Modification of zeolite H-Y by dithiane oxides (2-R-1,3-dithiane 1-oxide, R = H, CH3, C6H5) is shown to enhance significantly its activity for the acid-catalysed gas phase dehydration of butan-2-ol. The rate enhancement is observed for catalysts that are prepared using two commercial samples of zeolite H-Y. The origin of this effect is examined using in situ FTIR spectroscopy and Al-27 and Si-29 MAS NMR spectroscopy studies of the modified zeolite. The rate enhancement is considered to result from a specific interaction between the dithiane oxide modifier with both the extra-framework and framework aluminium in the zeolite.