The chemical bath deposition of metal sulfide, especially CdS, is the most common way of growing buffer layers in the chalcopyrite photovoltaic solar cells. From the kinetic point of view, the reactions should always be reproducible with the same initial reactant concentration of thiourea, ammonia, and cadmium salt, the same temperature, and the same deposition time. But different reaction kinetics of CdS molecule formation and deposition is observed. Different thiourea lots with nominally the same purity show different kinetics of the reaction and resulting CdS layer thicknesses after the reaction. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3458872] All rights reserved.