The molecular weight of pig liver carboxylesterase (~88% pure) has been determined as 163,000 (±15,000). The enzyme has two active sites per molecular weight of 163,000 as shown by titration with pnitrophenyl dimethylcarbamate and p-nitrophenyl diethyl phosphate. The enzyme undergoes slow irreversible inactivation at pH 5, but on dilution at pH 7.5, the enzyme dissociates apparently into half-molecules which are active. No evidence has been obtained for the dissociation of the enzyme into species of mol wt ~40,000, either at pH 8 or in 8 M urea.