A process for preparing a 3-isothiazolone of the formula ##STR1## wherein Y is selected from the group consisting of alkyl or substituted alkyl of 1 to 10 carbon atoms; unsubstituted or halogen-substituted alkenyl or alkynyl of 2 to 10 carbon atoms; and aralkyl or halogen-, lower alkyl-, or lower alkoxy-substituted aralkyl of up to 10 carbon atoms;X is hydrogen or a (C.sub.1 -C.sub.2) alkyl; andX.sup.1 is hydrogen, chlorine, or a (C.sub.1 -C.sub.2) alkyl; comprising(a) reacting anhydrous ammonia with an isothiazolone salt of the formula ##STR2## wherein Z is chlorine, bromine, sulfate or fluorosulfonate;m is 1 when Z is chlorine, bromine or fluorosulfonate, and m is 2 when Z is sulfate; and(b) separating the resultant (NH.sub.4).sub.m Z from the resultant free base isothiazolone is disclosed.