The mapping of steady-state diffusion layers in the vicinity of planar electrodes was investigated in presence of an excess and default of supporting electrolyte. The concentration profiles were fully characterized using an ultramicroelectrode as an amperometrie probe placed at several positions inside the diffusion layer. The dual influence of migration and convection on mass transport of electroactive species was evidenced experimentally through the study of a few one-electron electrochemical reactions. The specific distribution of the supporting electrolyte at the electrode interface was also performed upon detecting its cation purposely labelled by an electroactive group. The theoretical concentration profiles were evaluated on the basis of a model previously developed to account for the influence of natural convection on the building-up of diffusion layers. The agreement observed between data for the electrochemical reactions investigated validates a posteriori all the assumptions required to establish theoretically and experimentally the concentration profiles in default of supporting electrolyte. (c) 2006 Elsevier B.V. All rights reserved.