A composition comprising a microbicidally synergistic mixture of A) methylisothiazolinone (MIT) or benzisothiazolinone (BIT) with B) maleic acid wherein the synergistic ratio of MIT/ maleic acid range is from 1/50 to 1/300, and wherein the synergistic ratio of BIT/ maleic acid range is from 1/30 to 1/1000.