The anion-proton antiporter ClC-5 is expressed mainly in the kidney proximal tubules and is involved in the regulation of vesicle acidity along the endocytotic pathway. Genetic defects in the gene encoding for ClC-5 are associated with Dent's disease, a renal disorder characterized by tubular proteinuria, hypercalciuria, calcium nephrolithiasis, nephrocalcinosis and chronic renal failure. ClC transporters shuttle 2 Cl^- anions in exchange for 1 H⁺ across the membrane. The detailed mechanisms underlying this coupled antiport are still not well understood. Using whole-cell patch clamp combined with intracellular pH measurements we investigated the effects of the transport substrates - intracellular protons and extracellular anions on the transport characteristics of ClC-5. ClC-5 transport became uncoupled in external SCN^- resulting in ~14 fold increase of measured total currents when compared to external Cl^- at internal pH 7.4; surprisingly, proton transport was not altered significantly. Increased internal proton concentration (pH 6) led to much stronger (~50 fold) current increase in external SCN^- when compared to external Cl^-. However, proton transport was also increased indicating unchanged degree of uncoupling by SCN^-_ext. Mutating the so called "proton glutamate" E268 to histidine impaired the uncoupling effect of external SCN^- but also reduced the effects of acidic internal pH. Measurements of nonlinear voltage dependent capacitances associated with ClC-5 operation indicate that the observed effects of internal protons are not due to changes in the open probability of the depolarization activated gate as well in WT as in the E268H mutant. Our findings suggest a concerted interplay between internal protons and external anions in the regulation of the transport probability of ClC-5. The effects of the "proton glutamate" mutant E268H indicate thereby interdependence between gating and stoichiometric transport coupling in ClC transporters.