To obtain an aromatic nitrile by reacting an aldehyde with hydroxylamine sulfate under specific condition.

The objective nitrile of the formula II is produced by reacting an aldehyde of the formula I (X is CH or N; R1 and R2 are each H, Cl, Br, a 1-6C alkyl, a 1-6C alkoxy, a 1-6C alkyl-amino, CN, phenyl, phenoxy or dimethylamino or R1 and R2 together form a condensed benzene ring, etc.) with hydroxylamine sulfate and dehydrating the reaction product. The reaction is carried out in an organic solvent immiscible with water (e.g. toluene) in the presence of a t-amine base of the formula III (R3 and R4 are each H, methyl or ethyl), etc., at 100-160°C while distilling out water isolated by the reaction under a pressure of 0.02-1.5bar and removing the ammonium salt. The compound of the formula II is useful as a synthetic intermediate for a diketopyrrolopyrrole pigment.