Modification of zeolite H-Y by dithiane oxides is shown to enhance significantly the rate of dehydration of butan-2-ol to butenes. Reaction of racemic butan-2-ol over zeolite H-Y modified with (R)-1,3-dithiane-1-oxide indicates that the high activity catalyst is enantiomerically discriminating, as one enantiomer reacts preferentially to the other, although both are present within the micropores under the reaction conditions. The origins of the rate enhancement have been investigated spectroscopically using FTIR and MAS NMR, The enhanced activity is considered to result from a specific interaction between the sulfoxide modifier with the Bronsted acid sites associated with framework hydroxyl groups together with a strong interaction with an extra-framework aluminium species. This combination results in the formation of a rigid high activity site. Investigations using the amino acid cystine as an alternative to 1,3-dithiane-1-oxide have indicated that in this case the modifier is more weakly bound within the zeolite pores and exhibits considerable molecular motion, leading to the modified catalyst demonstrating no enantioselectivity or rate enhancement.