This paper aims mainly the characterization of water vapor desorption process and its variability under site conditions. It presents a probabilistic approach allowing the assessment of the distribution laws for the Water Vapor Desorption Isotherms (WVDI) and the moisture diffusion coefficient. These two parameters were evaluated from multistage drying experiment. For each stage, the analysis ofthe equilibrium state allows the determination of WVDI whereas the analysis of the desiccation kinetic makes possiblethe evaluation of the moisture diffusion coefficient. In order to have a representative variability, the study was carried outon two different High Performance Concretes (HPC) during a largescale construction site: overall, more than 300 specimens were tested. Results indicate that Gaussian distribution can be used for modeling the statistical variability of desorption isotherm while log-normal distribution is more appropriate for describing the variability of moisture diffusion coefficient. Furthermore, whatever is the concrete, the coefficient of variation on the water content at equilibrium remain very similar