A process for the preparation of a malonic acid compound of formula: in which R2 and R5 are identical or different and are a hydrogen atom, a C1-C4-alkyl group or an aryl group, comprising converting a compound of formula: in which R2 has the meaning mentioned above and R3 is ???CN or COOR4, in which R4 is a C1-C4-alkyl group, provided that when R2 is hydrogen, R3 is not ???CN, into the malonic ac