Experimental results obtained by combined in situ scanning probe microscopy (SPM) and electrochemical impedance spectroscopy (EIS) studies of underpotential deposition (UPD) of Pb on Ag(lll) and Ag(100) are correlated and interpreted in terms of the kinetics of local electrode processes. Exchange current densities at different underpotentials are related to different low-dimensional (1D and 2D) Pb phases directly observed by in situ SPM. First order phase transitions between expanded (gas-like) and condensed (liquid- or solid-like) 2D Pb phases as well as surface alloy formation processes are taken into account for the interpretation of impedance data, Relatively simple conditions are obtained on Ag(111) substrates where the formation of a condensed Pb monolayer starting exclusively at monatomic steps is observed by in situ SPM at low supersaturations with respect to the condensed 2D Pb phase. The analysis of SPM and EIS data indicates that the lateral growth of the condensed Pb monolayer preferentially occurs via a direct transfer of Pb2+ from the electrolyte to the step edges.