The hydrogen evolution reaction (her) was studied on two different surfaces obtained by electrodeposition of Ni and Hg on Pt ultramicroelectrodes. This reaction was analyzed in alkaline and acid media by a non-linear regression procedure together with mechanistic considerations. The derivations of the kinetic equations were based on a Volmer-Heyrovsky mechanism with a different rate determining step for each surface. On Pt/Ni electrodes an apparent activation energy of 56 kJ mol(-1) was obtained while on Pt/Hg this value increases up to 204 kJ mol(-1). Such difference was analyzed as a change in the nature of the activated complex formed as intermediate of the reaction. The greater affinity existing between H and Ni facilitates the adsorption step on Ni surfaces, resulting in the Heyrovsky step to be the rate determining one, with an activated complex of the type (Ni-H...H...OH)*. On Hg, the adsorption is more difficult and the Volmer step becomes determining with the species (Hg...H...H2O+)* being the activated complex. The rate constant values for each step were determined by a non-linear regression procedure. In this work, the utilization of ultramicroelectrodes and the special methodology used to conditioning the surface brought important contributions to the interpretation of experimental results by avoiding the influence of ohmic drop and bubbles evolution on the surface, thus increasing the reproducibility of the experiments.