Heterojunction cells of n-CdS0.5Se0.5/p-InP, fabricated by vacuum deposition of CdS0.5Se0.5 thin films into InP single crystals, show a conversion efficiency as high as 5% and an open circuit voltage of 0.78 V under illumination by light with a power density of 50 mW m(-2). The CdS0.5Se0.5 films, nominally 400 nm thick, have resistivities of the order of 10(-1) Omega cm, so that the film makes a good electrical contact between the junction and the front aluminium electrode. Measurements of J- V and C- V characteristics also have been evaluated to identify the mechanisms of barrier formation and current flow. At low voltages, the current in the forward direction varies exponentially with the voltage. At higher voltages, two distinct regions of ohmic and space-charge-limited conduction under forward bias are observed. The linearity of the C-2- V dependence is associated with a homogenous distribution of the impurities inside the space-charge region. Under a reverse bias, the conduction process at low voltage is determined by Schottky emission over a potential barrier of approximate height 0.88 eV and thickness 98 nm. At higher voltage levels, the Poole-Frenkel effect is observed.