Polybenzimidazole (PBI) - based membranes are one of the systems of choice for polymer electrolyte fuel cells. Monomer sulphonation is one of the strategies suggested to improve proton transport in these membranes. Here we report a NMR and dynamic mechanical study aiming to investigate the effect of the sulphonation on the proton dynamics and the mechanical properties of the membranes. The analyses of 1H self-diffusion coefficients and 1H and 31P spectra vs. temperature show that sulphonation causes the formation of inter-chain crosslinks, which involve phosphoric acid molecules and the sulphonic groups. This, in turn, reduces the proton mobility and, consequently, the ionic conductivity. The increase of the membranes stiffness with sulphonation is confirmed by dynamic mechanical analysis through the behaviour of the storage modulus