The mass sensing principle of quartz crystal has successfully been employed to study the diffusion of dimethylmethylphosphonate (DMMP) and water through a poly( dimethylsiloxane) (PDMS) membrane. For this study a new permeation cell has been fabricated which houses a Cu(II) butyrate ethylenediamine-coated quartz crystal sensor. This development was motivated by the desire to measure the diffusion coefficients rapidly, precisely, and at different temperatures. Temperature dependence diffusion coefficients of DMMP and water are also evaluated using the same cell, which enables calculation of the activation energies for the diffusion process. From the diffusion and solubility data permeability coefficients of DMMP and water have been calculated. It is observed that PDMS membrane offers a selectivity for DMMP over water roughly by 13 : 1 (i.e., P-DMMP/P-water > 13).