The human erythrocyte anion transport protein (AE1) mediates the rapid, tightly coupled, electroneutral transmembrane exchange of bicarbonate for chloride. AE1 transports a wide range of oxyanions, such as phosphate, sulfate and the physiological substrate bicarbonate. In this study, the transport characteristics of the selenium based oxyanions selenite (SeO3(2-)) and selenate (SeO4(2-)) were determined. The pH dependence of selenate influx was consistent with a titratable carrier having a extracellular pK value of 5.67 +2- 0.09. In contrast, the pH dependence of selenite influx had a maximum near pH 7.0, consistent with a hypothesis proposed by Labotka and Omachi (J. Biol. Chem. 263: 1166-1173, 1988) that the pH maximum of the transport of titratable anions is located at the midpoint between the pK of the carrier (5.7) and the pK of the titratable anion (8.3). Analysis of the transport rates and structures of these as well as a variety of other oxyanions reported in the literature suggested that oxyanions bind in a three-oxygen atom binding site, and that the formation of the transition state necessary for transport is sterically restrained by oxyanions that protrude in a direction perpendicular to the three-oxygen binding plane. This hypothesis can be used to predict the relationship between the transport rates of many oxyanions reported in the literature and should prove useful in helping to understand the molecular mechanism of AE1 mediated transport.