Acid ammonium sulphate and sulphuric acid are recovered from the residues from the manufacture of acids or esters of the acrylic group by sulphuric acid hydrolysis of nitriles or cyanhydrins which may be followed by esterification, whereby the acid ammonium sulphate contained in the said residues is crystallized out by cooling, the crystals being then separated from the mother solution containing unused sulphuric acid in excess together with residual organic constituents, the more or less carbonized heavy organic products are eliminated from the residues before or after cooling and separation of the crystals, and finally the recovered acid solution may be reconcentrated, if desired, for re-use in the hydrolysis reaction vessel. The crystallization of acid ammonium sulphate may be effected at temperatures between 10 DEG and 30 DEG C., and the crystals separated by filtration or pressing, e.g. in a centrifuge. To facilitate filtration, a part of the recovered acid solution may be taken off and fed back into the crystallizer. The recovered acid ammonium sulphate may be used for the manufacture of neutral ammonium sulphate. The carbonized heavy organic products may be eliminated from the residues by adsorbing them in hot or cold state, continuously or intermittently, on an adsorbent earth or carbon. The solution to be purified may be passed through a bed of adsorbent or the latter may be mixed with the solution and the suspension passed through a filter. Before the crystallization step and before or after the elimination of heavy organic products the residues may be evaporated under reduced pressure and the volatile organic products thereby separated. Alternatively, after the elimination of acid ammonium sulphate crystals and before treatment on the adsorbent, the recovered acid solution may be partly re-concentrated by heating in the presence of a small quantity of an oxidizing agent such as nitric acid. At the same time at least a part of the heavy organic products are cokified by this step and are later removed by passing over the adsorbent. The final reconcentration of the recovered acid solution may be effected by heating and/or by the addition of sulphuric acid or oleum. Flow diagrams, apparatus and detailed examples are described for the recovery of acid ammonium sulphate and sulphuric acid from the manufacture of ethyl acrylate by sulphuric acid hydrolysis of acrylic nitrile and esterification with ethanol, and from the manufacture of methyl methacrylate by sulphuric acid hydrolysis of acetone cyanhydrin and esterification with methanol.ALSO:The residues from the manufacture of acids or esters of the acrylic group <FORM:0815163/IV (b)/1> the free valencies of which are attached to hydrogen atoms or other radicals, by sulphuric acid hydrolysis of the corresponding nitriles or cyanhydrins which may be followed by esterification, are recovered by crystallizing out the acid ammonium sulphate contained in the residues by cooling, separating the crystals from the mother solution containing unused sulphuric acid in excess and almost the whole of the organic constituents of the initial residues, and eliminating the more or less carbonized heavy organic products from the residues before or after cooling and separation of the crystals, and the recovered acid solution is re-introduced into the hydrolysis reaction vessel, after reconcentration if necessary. The crystallization of acid ammonium sulphates may be effected at temperatures between 10 DEG and 30 DEG C., and the crystals separated by filtration or pressing. To facilitate filtration a part of the acid solution, before or after the adsorption treatment, may be taken off and recycled to the crystallizer. The carbonized heavy organic products may be eliminated by adsorbing them in hot or cold state, continuously or intermittently, on an adsorbent earth or carbon. The solution to be purified may be passed through a bed of adsorbent or the latter may be mixed with the solution and the suspension passed through a filter. Before the crystallization step, and before or after the elimination of heavy organic products, the residues may be evaporated under reduced pressure or under vacuum and the volatile organic products thereby separated, such as methacrylic acids or esters, intermediate products or alcohol in excess, may be recycled to the hydrolysis or esterification stages. Alternatively, after the elimination of ammonium bisulphate crystals and before treatment on the adsorbent, the recovered acid solution may be partly re-concentrated by heating in the presence of a small quantity of an oxidizing agent such as nitric acid. At the same time at least a part of the heavy organic products are cokified by this step and are later removed by passing over the adsorbent. The final reconcentration of the recovered acid solution may be effected by heating and/or by the addition of sulphuric acid or oleum. The re-use of this acid required for the manufacture of acrylic acid a concentration of 78 per cent and for the manufacture of methacrylic acid a concentration of 100 per cent. Flow diagrams, apparatus and detailed examples are described for the manufacture of ethyl acrylate by sulphuric hydrolysis of acrylic nitrile and esterification with ethanol, and for the manufacture of methyl methacrylate by sulphuric hydrolysis of acetone cyanhydrin and esterification with methanol.