PURPOSE: A kind of alkyl halide is used to form selectively the quarternary salt of isoquinoline from isoquinoline in a quinoline base mixture and the product is used as an anticorrosive agent directly or is decomposed to give isoquinoline.

CONSTITUTION: A quinoline base mixture such as a fraction boiling at 240W250°C which has been separated from coal tar is combined with an alkyl halide of the formula: RX (R is 1W20C substituted or unsubstituted alkyl; X is Cl, Br or I) such as lauryl chloride in an amount of 0.8W1.5 times the molar quantity of the isoquinoline in the base mixture and the reaction is effected at 100W250, preferably 100W200°C to form a quarternary alkylated compound of isoquinoline. The reaction mixture is cooled and stood to conduct the layer separation for isolation of the quarternary salt. When the fraction boiling at 240W250°C from coal tar is employed, the unreacted quinoline base is almost composed of methylquinoline, which is used as a raw material for quinophthalone pigments.