High-efficiency photoelectrochemical water-splitting devices require the integration of electrocatalysts (ECs) with light-absorbing semiconductors (SCs), but the energetics and charge-transfer processes at SC/EC interfaces are poorly understood. We fabricate model EC-coated single-crystal TiO2 electrodes and directly probe SC/EC interfaces in situ using two working electrodes to independently monitor and control the potential and current at both the SC and the EC. We discover that redox-active ion-permeable ECs such as Ni(OH)2 or NiOOH yield 'adaptive' SC/EC junctions where the effective Schottky barrier height changes in situ with the oxidation level of the EC. In contrast, dense, ion-impermeable IrOx ECs yield constant-barrier-height 'buried' junctions. Conversion of dense, thermally deposited NiOx on TiO2 into ion-permeable Ni(OH)2 or NiOOH correlated with increased apparent photovoltage and fill factor. These results provide new insight into the dynamic behaviour of SC/EC interfaces to guide the design of efficient SC/EC devices. They also illustrate a new class of adaptive semiconductor junctions.