PURPOSE: To prepare the subject compound free from salt and opptionally free from water with high purity and high yield by bringing hydrochloric acid solution of isothiazolone to contact with a basic anion exchange resin.

CONSTITUTION: A free basic isothiazolone is produced by treating the solution of an isothiazolone of formula I (Z is Cl, Br, sulfate or fluorosulfonate; (m) is 1 or 2) with a basic anion exchange resin, preferably a weak basic anion exchange resin to neutralize HmZ contained. The objective compound of formula II (Y is a 1-10 C alkyl, 2-10 C alkenyl, alkynyl, aralkyl having not more than 10 carbon atoms, a halogen, etc.; X is H or a 1-2 C alkyl; X1 is H, Cl or a 1-2 C alkyl) is obtained by binding HmZ on the resin and separating the solution containing the free basic isothiazolone from the resin used. Isothiazolones, especially isothiazolones free from its salts are useful as anti-microbial agents for the latices and corrosion labile aqueous systems.