The invention discloses a process for manufacturing a 5-formyl-pyridine-2,3-dicarboxylic acid ester (I) wherein Z is hydrogen or halogen; Z1 is hydrogen, halogen, cyano or nitro and R1, R2 are independently C1-C10-alkyl, comprising the steps of (i) reacting a compound of formula (II), wherein the symbols are as in formula (I), with a nitrosation agent (III) R3-O-N=O (III) wherein R3 is C1-C8-alkyl, in the presence of an alkali metal or alkaline earth metal alcoholates or carbonates in a polar aprotic solvent at a temperature of from -45 to 40 DEG C, to obtain an oxime compound (IV) where Z, Z1, R1 and R2 are as in formula (I), and (ii) reacting oxime compound (IV) with an aliphatic C1-C10-aldehyde in the presence of a Lewis acid at a temperature in the range of from 0 to 100 DEG C. The compounds of formula (I) are useful intermediates in the synthesis of herbicidal imidazolinones, like imazamox.