In situ atomic force microscopy (AFM) of the electrochemical deposition of cerium oxide coatings is presented for the first time. The AFM images and the tip-deposit interaction, observed during early stages of deposition, demonstrate clearly the presence of a transitional gel state of the depositing material between the preceding solution-dispersed state and the subsequent solid state. The deposition is shown to occur through a nucleation and growth mechanism in which microscopic nuclei crystallized out from this intermediate gel mass serve as the growth centres of the new phase. Among the successive steps of the overall process, including reactants discharge at the electrode surface, and mass transfers to and from it, the nucleation and growth process is the rate-determining step.