Nonhydrolyzable analogues of both stereoisomers of phosphotyrosine, and a series of related aryloxy (or thio) methyl and aryloxy (or thio) ethyl phosphonic acids of the general formula RX–(CH2)n–PO3H2 (where X = O or S and n = 1 or 2), have been tested as nonhydrolyzable mimetics of phosphatase substrates. These compounds were tested against a panel of phosphatases (two alkaline phosphatases, a protein–tyrosine phosphatase, and two serine/threonine phosphatases) with different active site motifs. The compounds exhibit competitive inhibition toward all enzymes tested, with the best inhibition expressed toward the Ser/Thr phosphatases. The stereoisomers of the phosphotyrosine analogues exhibited an unexpected difference in their inhibitory properties toward the protein–tyrosine phosphatase from Yersinia. The Ki for the d isomer is 33-fold lower than that of the l isomer, and is more than an order of magnitude lower than the reported Km of the substrate l-phosphotyrosine.