The present invention relates to a process for preparing pyridinyl and pyrimidinyl mono-fluorinated beta keto esters of formula (I): wherein: R1 represents a pyridine ring or a pyrimidine ring, the rings being optionally substituted by a C3-6 cycloalkyl group, a C1-4 alkyl group, a C1-4 alkoxy group, a benzyl group or a halogen atom; R2 represents a hydrogen atom, a C1-6 alkyl group or ahalogenatom; and R3 represents a C1-6 alkyl group; by reacting with fluorine a compound of formula (II) wherein R1, R2 and R3 have the same meaning as defined above.