The initial activity and selectivity of toluene chlorination by sulfuryl chloride was measured using dried and undried samples of four different zeolites : L, X, Y, and ZSM-5. In all cases, removing residual water from the zeolite increases its initial activity. The in-situ removal of water causes a large increase in the para-selectivity of zeolite L, a smaller increase for ZSM-5, and effectively no change for X and Y. The effect of water upon activity and selectivity can be attributed, at least in part, to the formation of aqueous HCl either via hydrolysis of sulfuryl chloride or as a byproduct of the chlorination reaction. Additional experiments show that the amounts of benzyl chloride and poly-chlorinated toluenes are reduced by excluding light and by using dried air to purge the reactor; both procedures reduce the generation of ree radicals which otherwise lead to the formation of these byproducts.