A biocide composition comprises a biocide and an adjuvant comprising a ketal having the formula (1)wherein a is 0 or an integer of 1 to 12, specifically 1 to 6, more specifically 1 to 4, still more specifically 2 to 4; b is 0, 1, or 2; R is —C(═O)OR1 or —CH2OH; R1 is a C1-18 alkyl or C5-8 cycloaliphatic or heterocyclic group; R2 is a divalent C1-8 alkylene group optionally substituted with up to 5 hydroxyl groups; R3 and R4 are each independently hydrogen or C1-6 alkyl; and R5 is hydrogen or C1-3 alkyl.