A photoelectrochemical study of SnO2/CdS/CdTe thin film heterostuctures in contact with an electrolyte has been carried out. Upon thermal annealing treatment the isotype structure SnO2(n)/CdS(n)/CdTe(n) is converted into an n/n/p structure, due to type conversion of the CdTe layer. This process is the basis of the formation of efficient CdS/CdTe solar cells. In contact with the electrolyte a transistor-like n/n/p/n structure is formed. A theoretical analysis of the behaviour of this structure is presented. By changing the applied potential the relative contributions from the internal CdS/CdTe barrier and the outer CdTe/electrolyte can be separated. The simulation of the spectral responses allows one to determine the semiconducting properties of the semiconductors involved and those of the internal barrier. The improvement of the p-type CdTe properties caused by a CdCl2 surface treatment before annealing is confirmed by this method.