The opto-electrical behaviour of single crystal n-type CdTe in aqueous solutions containing several redox couples Sn2+,4+, Fe2+3+ and Fe(CN)63-,4-in the dark and under irradiation is described. The minority carrier diffusion length was 0.13 µm from a combination of results on photocurrents and capacitance. The dopant concentration was 1.4 × 1017 cm-3 from Mott-Schottky plots. In the cdTe-Sn2+,4+ system the photovoltaic properties of the junction depended on the state of charge injection into the interface region. We distinguished between a 'high state' and a 'low state' where in the 'high state' both the open circuit photovoltage and the fill factor were approximately twice as large as in the 'low state'. Possibilities for a difference in the value of the built-in potential obtained from the photocurrent method and from a Mott-Schottky plot are described. The built-in potential (or the flat-band potential) was independent of the redox potential, an effect which is analogous to Fermi-level pinning. The empirical relationship between barrier height and band gap in a Fermi-level pinned junction held for our system.