Current transients recorded during deposition are generally used to characterize the nucleation and growth modes of films formed electrochemically. However, comparison with surface observations reveals that the information derived from transients is often rather inaccurate, particularly when a parallel reaction occurs (e.g. H-2 codeposition) or sites available for nucleation are inhomogeneously distributed over the surface. Two methods of determining the partial current corresponding to metal deposition are described. In the case of Pt electrocrystallization on n-GaAs, analysis of the transients is improved, but in certain cases is still unable to describe the details of the process (the actual film formation was elucidated using transmission electron microscopy). Correlations between the modes of nucleation of various metals, the metal reactivity with respect to each constituent of the substrate and the electrochemical process are analysed. This yields insights into the factors determining the mechanism of incorporation of adatoms on the GaAs surface.