Cl-(H2O)(n) (n = 1-6) clusters have been studied using ab initio calculations. The structures of the various Cl-(H2O)(n) isomers differed considerably from those of the corresponding F-(H2O)(n) isomers for n = 2 and 4-6, and especially so for n = 6. The relative stability of the Cl-(H2O)(n) isomers is also much different from that of the corresponding F-(H2O)(n) isomers at 0 and 298 K. The relative stability of the Cl-(H2O)(n) isomers at 298 K is different from that at 0 K, because of the entropy effect. The ionization potential, charge-transfer-to-solvent (CTTS) energy, and the OH stretching vibrational spectra are reported to facilitate future experimental work.