PURPOSE: To produce the titled compound by heating 2-hydroxy-4-substituted- isophthalaldehyde together with an alkylation agent in the presence of an alkali inwater or in a water-containing solvent, thereby carring out the removal of carbon monoxide and the alkylation at the same time.

CONSTITUTION: A diformyl compound such as 2-hydroxy-4-substituted-isophthalaldehyde, is subjected to the selective removal of one carbon monoxide molecule and the alkylation reaction. In the above process, 2-hydroxy-4-substituted-isophthalaldehyde of formula I (X is alkoxy, alkyl, halogen or H) is made to react with the alkylation agent of formula RX or R2SO4 (R is alkyl or aryl) in the presence of a base such as NaOH, KOH, etc. in water or a water-containing solvent to obtain the objective 2-alkoxy-4-substituted-benzaldehyde of formula II.

USE: An intermediate of pharmaceuticals and agricultural chemicals.