A composition comprising a microbicidally synergistic mixture of A) methylisothiazolinone (MIT) or benzisothiazolinone (BIT) with B) decylene glycol wherein the synergistic ratio of MIT/ decylene glycol range is from 1/0.8 to 1/50, and wherein the synergistic ratio of BIT/ decylene glycol range is from 1/50 to 1/133.