Diusivities within crystals of highly siliceous MFI-type zeolite in a binary component system were measured over a temperature range of 448Β±498 K and under a total pressure of 13.3 Pa by a newly developed DRP method using parans (n-heptane, n-octane) and aromatics (ortho-and meta-xylenes). The diusivities of the fast diusing molecules were strongly aected by the co-existence of slow diusing molecules. The diusion of the former decreased as the number of slow molecules increased and the intrinsic diusion rate of the slow molecules decreased. In contrast, the diusivities of the slow molecules were not aected by the co-existence of fast molecules. To account for the dierence in the diusivities in the single and binary component systems, the random walk simulation was conducted. The dierence in the diusivities simulated agreed well with the results found experimentally. An empirical equation is proposed for the simple prediction of the reduction of the diusivity of fast molecules in a binary components system by modifying the previously reported equation for the decrease in diusivity caused by coke deposition. The estimated value using the empirical equation agreed well with the experimental data and the data calculated by the random walk simulation.