The acylation of thiophene by butyryl chloride has been studied in the liquid phase in the presence of 12 catalysts: HZSM-5, H-mordenite, and HY (ultrastable Y) with various framework Si/Al ratios, various numbers of Bronsted and Lewis sites, and various amounts of nonframework aluminum. The numbers of Bronsted and Lewis acid sites were obtained by FT-LR using chemisorbed ammonia, as described elsewhere. There is a correlation between the initial rates of reaction and the number of Lewis acid sites, whereas there is no correlation between the initial rates and the number of Bronsted sites. The correlation with the number of Lewis sites may be considered significant, since it expands on more than one order of magnitude. Poisoning of the catalyst probably results from the oligomerization on butyryl chloride. No carboxylic acid has been detected in the course of the reaction. Na attempt has been made to determine the nature of the poisoning reaction.