The relevance of ac measurements for the study of developing films at solid-liquid interfaces is discussed. An electrical model is introduced, and the correspondence of each circuit element with a chemical or physical process is explained. Further details are discussed mostly by considering the spontaneous development of a solid film at a Zn/CrO3(aq) interface. It is shown that less straightforward ac behavior may be understood after modification of the general electrical model, based on supplementary information on the studied system. For the experimental system considered, the most important film growth characteristics are derived. (C) 2000 Academic Press.