Platinum supported on nanoporous carbon (NPC) is promising candidate for using as electrodes in proton exchange membrane fuel cells. Performance of the anode of a fuel cell is markedly influenced by the efficiency of the splitting of hydrogen atoms by platinum and the transference of the produced protons to the carbon support (spillover process). Consequently a better understanding of these elemental processes could prompt the improvement of the materials used. With this aim, a series of Pt/NPC samples varying the metallic content were studied by electron spin resonance (ESR) under controlled gas environment. This spectroscopic tool is especially suitable for the investigation of processes that involve transference of electrons. In the present case, all materials studied, including bare carbon, showed a very narrow signal (Hpp=1.5–3 gauss) at g=2.0028±0.0002 after activation in vacuum at 500–C. In the case of the pure carbon and for the samples with lower platinum content (lower than 0.2 wt%) signals are significantly asymmetric, and their intensity is scarcely affected by the introduction of hydrogen up to 500°C. In contrast the spin concentration experiences a significant increment when the samples with platinum loading comes in contact with hydrogen at temperatures in the 300–500°C range. Although the centres originating these signals are located in the carbon matrix, the present results emphasize the importance of platinum for hydrogen activation and electron transference.