The microstructures of CdTe layers electrodeposited from basic ammoniacal electrolytes were investigated by transmission electron microscopy and compared with those from a conventional acidic sulfate electrolyte. As-deposited CdTe layers with a nearly stoichiometric composition from a basic electrolyte with a relatively high Cd(II)/Te(IV) concentration ratio were composed of randomly oriented crystallites with a grain size of ca. 10 nm, while those from an acidic electrolyte consisted of defective columnar grains with a (111) preferred growth orientation. Deposit containing around 65 atom % Te from a basic electrolyte with a relatively low Cd(II)/Te(IV) concentration ratio was found to be amorphous, which was easily crystallized into a mixture of CdTe, elemental Te, and a trace amount of Te2O5 by electron beam illumination. By annealing at 300degreesC for 1 h in an Ar atmosphere, CdTe grains from the basic electrolyte grew by about a factor of four, but at the same time, nano-voids were formed at the grain boundaries. In contrast, CdTe grains from the acidic electrolyte did not grow by annealing under the same conditions. (C) 2004 The Electrochemical Society.