A process for preparing an alkali metal or alkaline earth metal salt of an α,β-ethylenically unsaturated carboxylic acid, wherein a) an alkene, carbon dioxide and a carboxylation catalyst are converted to an alkene/carbon dioxide/carboxylation catalyst adduct, b) the adduct is decomposed to release the carboxylation catalyst with an auxiliary base to give the auxiliary base salt of the α,β-ethylenically unsaturated carboxylic acid, c) the auxiliary base salt of the α,β-ethylenically unsaturated carboxylic acid is reacted to release the auxiliary base with an alkali metal or alkaline earth metal base to give the alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid. Salts of α,β-ethylenically unsaturated carboxylic acids, such as sodium acrylate in particular, are required in large amounts, for example, for production of water-absorbing resins.