Process for the specific fluorination of the phosphorus atom of a phosphonocinnamic compound of the general formula (II) or (II'): ##STR00001## X being selected from: an oxygen atom, a sulfur atom, R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 and R.sup.6 being selected from: H, an alkyl group, an aryl group, OH, O-alkyl, S-alkyl, NH.sub.2, NH-alkyl, N-(alkyl).sub.2; R.sup.7 being selected from: H, an alkyl group, an aryl group, a silyl group; R.sup.8 being selected from: H, an alkyl group, an aryl group, TBDPS representing a tert-butyldiphenylsilyl group; in the process the compound is reacted with a fluorinating agent comprising a complex formed between a tertiary amine and hydrogen fluoride.