Thermoplastic compositions comprise a thermoplastic ether of cellulose and not more than 3 per cent by weight (based on the weight of cellulose ether) of a tri-ester of phosphorous acid. The ester may be symmetrical or mixed, e.g. tri-methyl-phosphite, tri-ethyl-phosphite, tri - n - propyl - phosphite, tri - iso - propyl phosphite, tri-n-butyl-phosphite, tri-iso-butyl-phosphite, tri-ethoxyethyl-phosphite, tri-cyclohexyl-phosphite, tri-menthyl-phosphite, tri-phenyl-phosphite, tri-o-cresyl-phosphite, tri-m-cresyl-phosphite, tri-p-cresyl-phosphite, the tri-xenyl-phosphites, tri-1-naphthyl-phosphite, tri - 2 - naphthyl - phosphite, tri - (2,4 - dibrom - 1 - naphthyl) - phosphite, tri - (1,6 - dibrom - 2 - naphthyl) - phosphite, tri - 1 - anthranyl - phosphite, o - xenyl - di - phenyl - phosphite, p - tertiary - butyl - di - phenyl - phosphite, o - chloro - phenyl - diphenyl - phosphite, di - o - chloro - phenyl - carvacryl - phosphite, di - phenyl - propyl - phosphite, di - phenyl - n - butyl - phosphite, and tri - (p - tertiary - amyl - phenyl) - phosphite. Tri - aryl phosphites in which the aryl group is a benzene or alkyl-substituted benzene nucleus in which the alkyl group is a straight or branched chain or a tertiary alkyl group are preferred. Plasticizers such as dibutyl phthalate, n-butyl stearate, tri-cresyl phosphate, tri-phenyl phosphate, di-butyl sebacate, mixtures of the dimer, trimer and tetramer of polymerized styrene, or partially hydrogenated isomeric terphenyls may be present in the compositions. Lubricant plasticizers, such as refined mineral oil, may also be present. The cellulose ether may be ethyl cellulose having an ethoxyl value of 44-49 per cent. The phosphite tri-esters may be incorporated in the cellulose ether compositions by mixing them with ethyl cellulose in flake form prior to colloidalization, they may be dissolved or suspended in the plasticizers employed and the solution thus obtained mixed with ethyl cellulose, or they may be mixed with solutions of ethyl cellulose in volatile solvents such as may be employed in the preparation of films by casting. Alternatively, the esters may be added directly to a plastic mixture of cellulose ether and plasticizing agent while it is being colloidalized at elevated temperatures on hot rolls, or in a mill or mixer. The invention may be applied to the preparation of moulding and extrusion compositions and of films, foil, sheets, rods and tubes made by methods employing solvents and lower temperatures than prevail in moulding or extrusion. In an example, five ethyl cellulose compositions are prepared by mixing 100 parts by weight of ethyl cellulose (ethoxyl value 44.6 per cent) with 15 parts of dibutyl-phthalate and to each of four of these compositions are added 1, 2 and 3 parts of tri-cresyl-phosphite and 2 parts of tri-phenyl-phosphite respectively, while no phosphite is added to the fifth. The mixtures are dried, moulded at 200 DEG C. to form discs and the "yellowness coefficient" of the discs compared. Other examples are given of the preparation of moulding powders from ethyl cellulose, the same and other plasticizers and tri-phenyl, tri-cresyl, and p-tertiary-amyl-phenyl phosphites. In some cases the moulding powder (either baked or unbaked) is moulded into discs and data furnished for their "yellowness coefficient," the viscosities of comparable solutions prepared from the discs and the period required to produce crazing of the surface of the discs when exposed to "Weatherometer" tests. In two of the examples data are also given for notched Izod impact strengths of moulded bars before and after ageing at 140 DEG F. In a further example, a film is prepared by casting a solution consisting of 100 parts by weight of ethyl cellulose, 1 of dibutyl-phthalate and 1 of tri-cresyl-phosphite dissolved in 300 parts by weight of an 80/20 by volume mixture of toluene and ethyl alcohol. The degree of polymerization retained after ageing in oxygen at 200 DEG F. for 120 and 420 hours is estimated by measuring the viscosities of comparable dilute solutions of the material and compared with that of films prepared from solutions containing no phosphite. The addition of up to 3 per cent of the phosphite is said to increase the stability and resistance to the action of heat, light and air of cellulose ether compositions, particularly with respect to colour retention, brittleness and surface changes.