Members of the CLC family comprise Cl^– channels and Cl^–/H⁺ antiporters that conduct outwardly, inwardly, and non-rectifying currents. They also have different surface and organelle membrane localisation. Mammalian CLC-5 is a Cl^–/H⁺ antiporter [1, 2] that is expressed in the kidney, the proximal tubule in particular, and is involved in the process of albumin reabsorption from the glomerular filtrate. CLC-5 is thought to provide a conduit through which Cl^– can flow from the cytosol into the endosome to balance the H⁺ pumped by the H⁺-ATPase to acidify the contents: an electrical shunt. Various mutations in human CLC-5 result in Dent’s disease, which is characterised by the appearance of low molecular weight protein in the urine. We investigated the properties of CLC-5 by overexpressing EYFP-CLC-5 in human embryonic kidney cells (HEK293). Confocal imaging showed that CLC-5 was localised to both surface and endosomal membranes, consistent with CLC-5 immunuhistochemistry in native tissue [3]. Membrane currents were recorded by whole cell patch clamp using CsCl-based solutions, holding the cell at -30mV and applying 10ms steps from -100 to +200mV. Large and sustained outwardly rectifying currents were recorded at potentials positive to +30mV with fast activation (1ms) kinetics. Tail currents were recorded over the same voltage range following a prepulse to +200mV. At voltages that induced an outward current these currents were sustained, but inward currents were followed by a rapid (0.2ms) and complete deactivation. Extrapolating these properties to endosomal membranes this means that Cl^– ions may only flow from the endosome into the cytosol, coupled with the transport of H⁺ into the endosome. To account for changes during endosomal ionic homeostasis and acidification the biophysical properties were investigated using a range of Cl^– and H⁺ gradients. CLC-5 exhibited strong outward rectification under all conditions tested. The inhibition of CLC-5 by reduced extracellular pH could be explained by a shift in activation kinetics that is related to the Cl^–/H⁺ coupling. Only mutating the “gating glutamate” (E211A), which uncouples H⁺ transport, removed rectification and permitted the flow of currents in the opposite direction. Our results suggest that CLC-5 has both channel and transporter-like properties, has a non-conducting role at the surface membrane, and does not provide an electrical shunt that facilitates endosomal acidification by H⁺-ATPase. Instead CLC-5 may be involved in a complex of membrane proteins involved in endocytosis [4] and could provide a parallel endosomal acidification mechanism by exchanging Cl^– for H⁺ in early endocytotic vesicles.