The mechanism of chemical bath deposition of cadmium sulfide thin films from the ammonia-thiourea system is studied in situ by means of quartz crystal microbalance technique (QCM). The influence of reaction parameters (concentration of reactants, pH, anions, temperature, stirring rate) is determined. The growth is thermally activated with an activation energy of about 85 kJ/mol, which probably corresponds to a chemical step related to the decomposition of thiourea. The results are well interpreted by assuming an atom-by-atom growth mechanism. A model is presented, which fits most of the experimental results quantitatively. It involves two or three rate-limiting surface steps, the formation of CdS taking place via a surface complex between thiourea and cadmium hydroxide. Analytical expressions are given, allowing prediction of the rate under various conditions in this system.