Acrylic acid or methacrylic acid is obtained by reacting at an elevated temperature in the vapour phase acrolein or methacrolein respectively, or a compound which gives rise to acrolein or methacrolein under the reaction conditions, with oxygen over an oxidising catalyst comprising (i) a mixture of the oxides of molybdenum and cobalt and/or (ii) a compound of molybdenum, cobalt and oxygen. The catalyst may be deposited on a support, such as silica or alumina, and may contain minor quantities of tellurium oxides. Propylene or isobutene may be used as starting materials and these are converted to acrolein or methacrolein respectively under the reaction conditions. Preferably the reaction is carried out in the presence, as diluent, of a gas which is substantially inert under the reaction conditions, e.g. nitrogen, propane, butane, isobutane, carbon dioxide and steam. Preferably steam, nitrogen or mixtures thereof are used. The preferred temperature range for carrying out the reaction is 300-500 DEG C. Specifications 625,330, 648,386 and 878,803 are referred to.