Several complex naphthol AS-phosphates were found to be reagents for the demonstration of alkaline and acid phosphatase in frozen sections which had been briefly fixed in acetone or diacetone alcohol. The technique offers a relatively simple way of obtaining useful histochemical localizations with routinely cut frozen sections. These newer substituted AS-phosphates were found to be distinctly superior to unsubstituted naphthol AS-phosphate and are also intended to replace the previous simple naphthyl phosphate substrates. The substrates, which usually contained halogen groupings, were rapidly hydrolyzed (provided that they are properly synthesized) under histochemical conditions and yielded sharp localizations. Several solvents hitherto unde-scribed for this purpose were introduced as histochemical reagents. These included dimethyl-sulfoxide, bis[2-(2-methoxyethoxy)ethyl]ether, N-methyl-2-pyrrolidone, 2-pyrrolidone and butyrolactone. For alkaline phosphatase, phosphates of naphthols AS-E, AS-KB, AS-CL, and AS-AN were found to be useful. Naphthol AS-E phosphate was found to be especially useful for the study of leukocytes in blood films. For the demonstration of acid phosphatase naphthol AS-BI, AS-CL and AS-AN phosphates were of value.