The diffusion mechanism and the concentration dependence of the mass transfer rate of water vapor adsorbing in the zirconium-based metal organic framework (MOF) UiO-66 has been determined using a concentration swing frequency response (CSFR) apparatus. The mechanism that best describes the adsorption of water in UiO-66 is micropore diffusion and the rate is dependent on the gas phase concentration of the water. Moreover, the results show that as the gas phase water concentration increases, a minimum in the water adsorption rate occurs near 4 mol/kg of water loading. Although the Darken relationship is unable to predict the concentration dependence, it agreed qualitatively with the experimental diffusivity data extracted from the CSFR analysis. The results demonstrate the ability of the concentration-swing frequency response technique to accurately determine adsorption rate mechanisms and quantify the adsorption kinetics of a slow to moderately fast diffusing component in MOFs.