In the process for producing an unsaturated carboxylic acid by oxidation of an unsaturated aldehyde by treating the unsaturated aldehyde with a peroxide in the presence of selenium dioxide and/or tellurium dioxide (see Group IV (b)) some of the unsaturated acid product may be obtained in polymeric form particularly if an appreciable amount of water is present in the reaction mixture. The reaction mixture may be esterified with an alcohol by using the latter as solvent for the oxidation reaction and then adding an esterification catalyst such as sulphuric acid or p-toluene sulphonic and heating the mixture to remove the water formed. The polymeric acid may also be esterified by heating with an alcohol and an esterification catalyst after the unsaturated carboxylic acid has been distilled off. When water is used as the sole diluent for the oxidation reaction large amounts of the polymeric acids are generally formed. The unsaturated aldehydes to be oxidized may be aliphatic, alicyclic or aromatic and may contain substituents such as hydroxy, ether, carboxylic acid, carboxylic acid ester, keto, and nitro groups, or halogen atoms. The preferred peroxide is hydrogen peroxide but other suitable inorganic peroxides are sodium and barium peroxides. Suitable organic peroxides are tertiary-butyl hydroperoxide, benzoyl peroxide and per-acids such as peracetic, performic, and perphthalic acids. Mixed peroxides obtainable by partial oxidation of hydrocarbons, e.g. as described in Specification 540,534, [Group III], may also be used. In an example acrolein is oxidized with an aqueous solution of hydrogen peroxide in the presence of selenium dioxide using water as diluent, high boiling polymerized products of acrylic acid being obtained.ALSO:An unsaturated carboxylic acid is obtained by oxidation of an unsaturated aldehyde by treating the unsaturated aldehyde with a peroxide in the presence of selenium dioxide and/or tellurium dioxide. The preferred peroxide is hydrogen peroxide but other suitable inorganic peroxides which may be used are sodium and barium peroxides while suitable organic peroxides are tertiary butyl hydroperoxide, benzoyl peroxide and per-acids such as peracetic, performic, and perphthalic acids. Mixed peroxides obtainable by partial oxidation of hydrocarbons, e.g. as described in Specification 540,534, [Group III], may also be used. Anhydrous peroxide can be used but solutions, especially aqueous solutions, are preferred. Aqueous hydrogen peroxide solutions of 85-93 per cent concentration are suitable and between 0.5 and 1.0 mol. of hydrogen peroxide is preferably employed per mol. of unsaturated aldehyde. The amount of selenium dioxide and/or tellurium dioxide is generally at least 0.2 per cent and preferably at least 2 per cent based on the weight of the aldehyde. The reaction is preferably carried out at between 20 DEG C. and 100 DEG C. and atmospheric, superatmospheric, or subatmospheric pressure may be employed. The process is preferably carried out in a liquid medium comprising an organic solvent for the unsaturated carboxylic acid produced and preferably also for the unsaturated aldehyde used. Water may be present in limited amounts and in order to avoid polymerization of the acids the total amount of water present should not exceed 30 per cent by weight and more preferably not exceed 10 per cent by weight of the reaction mixture. The organic solvent may be a reactive material which is converted under the reaction conditions provided such conversion does not adversely affect the oxidation of the aldehyde but inert solvents are preferred. Suitable solvents include various alcohols, especially those having from 3 to 10 carbon atoms in their molecules, polyhydric alcohols, substituted alcohols, e.g. monoethers of dihydric alcohols, ethers, ketones, carboxylic acids, carboxylic acid esters, chlorinated hydrocarbons, aromatic hydrocarbons, and normally liquid aliphatic hydrocarbons. Hydrocarbons which are gaseous under normal conditions may also be used if the oxidation is carried out under sufficient pressure to keep them at least partly in the liquid state. The reaction can be carried out batchwise, intermittently or continuously. The solution from the oxidation reaction may be used for further synthesis without isolating the acid from the mixture. Thus when the acid is to be esterified with an alcohol the latter can be used as the solvent and on completion of the oxidation a small amount of an esterification catalyst such as sulphuric acid or p-toluene sulphonic acid is added and the mixture heated to remove the water formed. The unsaturated aldehydes to be oxidized may be aliphatic, alicyclic or aromatic aldehydes and may be substituted, e.g. by hydroxy, ether, carboxylic, carboxylic ester, keto, and nitro groups or by halogen atoms. The aldehyde is preferably one having one olefinic double bond, e.g. an alpha, beta-olefinic aldehyde such as acrolein or methacrolein but acetylenic aldehydes may also be oxidized to the corresponding acetylenic acids. Mixtures of two or more unsaturated aldehydes may be used. The reaction mixture may contain impurities such as saturated aldehydes. If a high ratio of peroxide to unsaturated aldehyde is employed further oxidation of the unsaturated acid to a hydroxy-unsaturated carboxylic acid may occur. In examples (1), (3) and (4) acrolein is oxidized to acrylic acid using tertiary butyl alcohol as solvent; when water is used as diluent in (1) polymeric products are obtained (see Group IV (a)); (2) methacrolein is oxidized to methacrylic acid using tertiary butyl alcohol as diluent; (5) alpha-chloracrolein is oxidized to alpha-chloracrylic acid using tertiary butyl alcohol as solvent; (6) acrolein is oxidized using n-butanol as solvent and a small amount of p-toluene sulphonic acid added to the reaction product mixture and the latter distilled to remove water azeotropically to yield n-butyl acrylate; instead of using n-butanol ethanol, sec.-butanol and allyl alcohol may be used to yield the corresponding acrylic esters; (7) tetrahydrobenzaldehyde (the adduct of butadiene and acrolein) is oxidized to tetrahydrobenzoic acid. In each example the oxidation is effected with aqueous hydrogen peroxide and selenium dioxide. Example 4 also refers to using tert.-amyl alcohol, dioxane, acetone, isopropyl alcohol, benzene, methanol and acetic acid as solvent and example 5 also refers to the oxidation of dibromaleic dialdehyde to dibromomaleic acid, of alphachloroglutaconic dialdehyde to alpha-chloro-glutaconic acid, of croton-aldehyde to crotonic acid, of tiglic aldehyde to tiglic acid, of oleyl aldehyde to oleic acid, of propargyl aldehyde to propargylic acid and of cinnamyl aldehyde to cinnamic acid.