Knowledge of the diffusivity of CO2 and 02 is of considerable importance for a quantitative assessment of the carbonation and corrosion of the reinforcement in concrete. A special measuring cell which allows the simultaneous determination of the effective diffusion coefficients of the two gases as a function of the relative humidity has been developed. Measurements have been carded out on carbonated discs of hydrated cement paste prepared with water/cement ratios between 0.4 and 0.8. The results show that, if the water/cement ratio is increased from 0.4 to 0.8, diffusion coefficients increase more than ten times. On the other hand, only little influence of the relative humidity between 50-90% the diffusivity has been observed on samples in adsorption equilibrium. The variation of diffusivity as a function of water content and porosity is explained by the characteristic microstructure, which has been characterized by water adsorption isotherms and mercury intrusion porosimetry measurements. Finally, a model with two levels in the microstructure is proposed to describe CO2 diffusion in a carbonating material.