The properties of the violet-colored acid phosphatase [EC 3.1.3.2] isolated from sweet potato tubers have been investigated in more detail. The enzyme catalyzed the hydrolysis of a wide variety of phosphorylated compounds including phospho-monoesters, nucleotide mono-, di-, and triphosphates, and inorganic pyrophosphate. No activity was detected for NAD+ and diphenyl phosphate. The optimal pH of hydrolysis was found to be approximately 5.8. The Km value determined with -nitrophenyl phosphate, the most active substrate so far examined, was 6.8lO-5M. Orthophosphate, arsenate, and molybdate inhibited the enzyme activity competitively with -nitrophenyl phosphate, whereas fluoride acted as an uncompetitive inhibitor. The enzyme was also inactivated by prior incubation with chelating agents such as o-phenanthroline and α, ά-dipyridyl. Restoration of the activity after inactivation was observed following the addition of Znt2+, Co2+, or Mn2+. Preincubation of the enzyme with Hg2+ caused rapid activation to a level as high as about 3 times the original activity. The substrate specificity and the stability of the Hg2+-treated enzyme were found to be significantly different from those of the native enzyme. This enzyme, however, had essentially the same pH optimum as the native enzyme. The amino acid composition of the violet-colored acid phosphatase was also determined.