We used Monte Carlo simulations to understand how the Si/Al ratio of MFI-type zeolites affects their adsorption performance in the removal of VOCs from water. We considered adsorption of methyl tertiary butyl ether (MTBE) and trichloroethylene (TCE) to zeolites because they have different interactions with water, with MTBE being hydrophilic and TCE being hydrophobic. We carried out adsorption simulation in MFI-type zeolites of three different Si/Al ratios (infinity, 191, and 95). Our results showed that because MTBE molecules can adsorb only to the channel intersection, the presence of Al sites close to the intersections would promote water clustering and prevent the access of MTBE molecules to these sites. However, if the hydrophilic sites are far from the intersections, MTBE adsorption capacity is less impeded. Therefore, the distribution of Al sites in ZSM-5 zeolites is also an important factor determining the MTBE removal efficiency of these materials. The reduction in TCE removal capacity of the zeolites due to increasing Al content is less severe because of the strong affinity of the zeolites to hydrophobic TCE with respect to water. Therefore, increasing TCE amount in the feed mixture leads to water exclusion in the zeolites.