Members of the SLC20 family or type III Na⁺-coupled P_i cotransporters (PiT-1, PiT-2) are ubiquitously expressed in mammalian tissue and are thought to perform a housekeeping function for intracellular P_i homeostasis. Previous studies have shown that PiT-1 and PiT-2 mediate electrogenic P_i cotransport when expressed in Xenopus oocytes, but only limited kinetic characterisations were made. To address this shortcoming, we performed a detailed analysis of SLC20 transport function. Three SLC20 clones (Xenopus PiT-1, human PiT-1 and PiT-2) were expressed in Xenopus oocytes. Each clone gave robust Na⁺ dependent ³²P_i uptake, but only Xenopus PiT-1 showed sufficient activity for complete kinetic characterization using two-electrode voltage clamp and radionuclide uptake. Transport activity was also documented with Li⁺ substituted for Na⁺. The dependence of the P_i-induced current on [P_i] was Michaelian and the dependence on [Na⁺] indicated weak cooperativity. The dependence on external pH was unique: the apparent P_i affinity constant showed a minimum in the pH range 6.2-6.8 of ~0.05 mM and increased to ~0.2 mM at pH 5.0 and pH 8.0. Xenopus PiT-1 stoichiometry was determined by dual ²²Na, ³²P_i uptake and suggested a 2:1 Na⁺:P_i stoichiometry. A correlation of ³²P_i uptake and net charge movement indicated one charge translocation per P_i. Changes in oocyte surface pH were consistent with transport of monovalent P_i. Based on the kinetics of substrate interdependence, we propose an ordered binding scheme of Na⁺:H₂PO₄^-:Na⁺. Significantly, in contrast to type II Na⁺/P_i cotransporters, the transport inhibitor phosphonoformic acid (PFA) did not inhibit PiT-1 or PiT-2 activity.