In a process for preparing an alkali metal or alkaline earth metal salt of an α,β-ethylenically unsaturated carboxylic acid, a) a transition metal-alkene complex is reacted with CO2 to give a metallalactone, b) the metallalactone is reacted with a base to give an adduct of the alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid with the transition metal complex, and c) the adduct is reacted with an alkene to release the alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid and regenerate the transition metal-alkene complex. The base is selected from alkali metal or alkaline earth metal hydroxides and alkali metal or alkaline earth metal superbases. The alkene is, for example, ethene. The transition metal complex comprises, for example, nickel and a bidentate P,P, P,N, P,O or P,carbene ligand, such as 1,2-bis(di-tert-butylphosphino)ethane.