An appropriate clamping pressure is required to ensure the gas-tight operation of a PEMFC assembly and to improve the contacts (i.e. mechanical, thermal, electrical) between its components. However, an excessive mechanical load may also worsen the cell performance, in particular through the reduction in the porosity and mass transport ability of the Gas Diffusion Layers. In this study, the effects of mechanical compression on the global performance of a 225 cm² PEMFC assembly are investigated by implementing cell voltage monitoring and polarisation curve measurements. The investigations are carried out with gradual increase / decrease and randomised load compression protocols applied using a specially designed mechanical compression unit. 12 levels of mechanical compression are considered, ranging from 0.35 to 2 MPa with steps of 0.15 MPa. The results of the characterisation techniques show that the PEMFC performance is improved at all tested operating conditions for mechanical compression up to 1.55 MPa. This finding is attributed to 2 the dominant reduction of the ohmic drop against the increase of the gas diffusion losses. It is also shown that compressing the PEMFC beyond 1.55 MPa would not lead to any further improvement of the global cell voltage output. This may even worsen the cell electrical characteristics by affecting its mass transport issues.