Electrodeposition of Pt nanoparticles under potentiostatic conditions was performed on several types of carbon electrode supports: commercial macroporous carbon (a three-dimensional electrode), glassy carbon and graphite. Conducting polymers (poly-aniline and poly-oaminophenol) were also used. The platinum nanoparticles were obtained by different Potential Step Deposition (PSD) methods in 5 mM H2PtCl6 + 0.5 M H2SO4 aqueous solutions. The effect of the final potential, time and number of steps on the quantity, distribution and size of the platinum nanoparticles was analysed. The mechanism of the electrochemical deposition of platinum was studied through the application of theoretical modelling. The progressive nucleation mechanism provided the closest agreement with the results obtained. In addition, the chemical state and morphology of the electrodeposited materials were determined by means of SEM, TEM and XPS. The results show that the carbon material structure has a strong influence on the Pt particle structure and this, in turn, affects the catalytic activity. ; Spanish Ministerio de Educación y Ciencia (Projects CTQ2006-08958/PPQ and MAT2004-1479) and the EU (FEDER).