The microcrack distribution in a low porous granite exposed to weathering after quarrying was investigated in order to detect its microstructural evolution. Two blocks (A and B) were taken in the same granite body from two quarry faces respectively exposed during a few weeks and twenty years. Porosimetry by injection of carbon-14-methylmethacrylate (14C-MMA) was used for analysing the porous medium. Measurements of optical densities of autoradiographs provided data for the localization and the quantification of porosity. Results are compared with those obtained by mercury porosimetry. Average porosity of sample A was found to be 0.37 ± 0.03% whereas average porosity in sample B reaches 1.07 ± 0.45%. Most of the porosity increase may be explained by the opening of grain boundaries.