A battery-free wireless sensor dedicated to water diffusion monitoring in dielectric materials is presented. It is applied here to the monitoring of water diffusion in organic coatings. The method is based on the variation of the radiofrequency (RF) wave propagation in an open microstrip structure covered by the coating under consideration when water diffusion occurs. Simulations and experimental characterization of electromagnetic wave propagation in a microstip line and a planar stub resonator are described. The results achieved on the resonator prove unambiguously the feasibility of the method. As a consequence, the proposed sensor should be considered as a promising and reliable tool for civil engineering applications.