The effectiveness of window blocking in reducing the diffusion of a sorbate inside a three dimensionally connected zeolite pore system is investigated using pulsed field gradient(PFG) NMR. H-1 PFG NMR measurements were used to study the diffusion of methane in two binary sorbate systems at varying cosorbate loading : (i) methane and benzene coadsorbed in NaY, and (ii) methane and ethylene coadsorbed in NaY. In both cases (i.e. benzene and ethylene), D-blacking molecule/D-methane was less than similar to 3 x 10(-2), and we are able to test percolation theory predictions. These measurements were compared with previously determined variable loading single-component methane self-diffusion coefficients in NaX (Karger et al., 1980, J. Chern. Sec. Faraday Trans. 176, 717-737). Large crystals of NaY (similar to 30 mu m) were essential for this study to ensure the accuracy of the diffusion measurements. A constant base methane loading of -2 molecules per NaY supercage was employed in all the samples used in this study. The methane self-diffusion coefficient in the presence of coadsorbed benzene and ethylene was reduced in a manner qualitatively consistent with concepts from percolation theory applied to a diamond lattice (representing the connectivity of the NaY supercages). Coadsorbed benzene though was more effective in "blocking" the methane diffusion. Excellent agreement was found between the measured methane selfdiffusion coefficients in the presence of benzene and the prediction based on the effective medium approximation (EMA) to the percolation theory. Suggestions are made about the prospects of applying other models to blocking by more mobile cosorbates like ethylene.