Acetate kinase, a member of the ASKHA (Acetate and Sugar Kinases, Hsp70, Actin) phosphotransferase superfamily is a central enzyme in prokaryotic carbon and energy metabolism. Recently extensive structural and biochemical studies of acetate kinase and related carboxylate kinases have been conducted. Analysis of the kinetic properties of wild-type and mutant enzymes has been impeded by the nature of the current assays for acetate kinase activity. These assays have the disadvantages of being either discontinuous or insensitive or of utilizing compounds that interfere with activity measurements. We have developed a novel continuous assay that depends on the purine nucleoside phosphorylase-based spectroscopic measurement of the inorganic phosphate generated by hydroxylaminolysis of one of the products of the acetate kinase reaction, acetyl phosphate. This assay has enabled a determination of the kinetic parameters of the Thermotoga maritima acetate kinase that indicates a lower K(m) for acetate than previously published.