This study presents a reliable method for evaluating chloride-effective diffusivity in cementitous materials. Four types of mortars with various w/c ratios and admixtures were investigated. Binding capacities of mortars were measured by solution immersion tests in order to respect non-linearity of binding in solving Fick’s diffusion equation. Total chloride profiles were precisely measured after diffusion tests at various times of exposure. Then free chloride profiles were evaluated, with full knowledge of isotherms and physical properties. From total and free chloride profiles, it was possible to calculate the evolution of chloride diffusivity over the whole range of chloride concentration. Diffusivity was found to decrease while free chloride concentration increases, but the influence is significant only for very low concentrations.