PURPOSE: To prepare the titled compound useful as an intermediate of pharmaceuticals and agricultural chemicals, in high yield, using a catalytic amount of acid, by reacting an aldoxime in a heated reactor in the presence of an acid catalyst, and distilling out the produced water from the system by the azeotropic distillation using an azeotropic solvent.

CONSTITUTION: The nitrile of formula II can be prepared from the aldoxime of formulaI(R is substituted alkyl, substituted alkenyl, substituted alkynyl, etc.), by contacting the aldoxime with an acid catalyst (e.g. H2SO4, HCl, nitric acid, etc.) in a heated reactor, and distilling out the produced water during the reaction from the reaction system together with a solvent which forms an azeotropic mixture with water (e.g. benzene, toluene, xylene, etc.). The dehydration reaction can be carried out in high yield by using a catalytic amount of acid. When a small amount of amide, especially formamide, is added to the reaction system, the amide acts as a catalyst and the dehydration proceeds without using an acid catalyst in an improved selectivity.