Phosphating is the most commonly applied pre-treatment process. However, some disadvantages exist: heavy metals and phosphoric acid are used in the process. Volkland et al. developed a process, in which these compounds can be left out, using bacteria to activate the phosphating process. This paper addresses the analysis of this phosphating process using electrochemical impedance spectroscopy. EIS-measurements are carried out during 150 h to understand the electrochemical processes taking place during the phosphate formation. A model is derived that can explain the processes. The process appears to be a two-step adsorption process. The first step is formation of a thin layer of bacterial products. Formation of a dense layer of iron phosphates as a second step takes place within 2 days. © 2002 Elsevier Science Ltd. All rights reserved.