Activity and selectivity in the cracking of 2,3-dimethylbutane on steam-stabilized ultrastable HY (USHY), and on the same steamed USHY after extraction using (NH4)2 SiF6, have been studied at 400-degrees-C. Based on the data presented here, a detailed mechanism is proposed for the cracking of 2,3-dimethylbutane on the two catalysts. The mechanism accounts for the kinetics and the selectivity behaviour observed and shows that extra-framework aluminum (EFAI) does not participate in the cracking of 2,3-dimethylbutane under our experimental conditions. Kinetic parameters obtained from the fit of experimental conversion data, and the initial product selectivities, show that monomolecular and bimolecular reactions are active on both catalysts. These quantitative measures show that extraction of EFAI from the steamed catalyst results in a small increase in the total initial rate of reaction. We take this to indicate that new sites are activated by the removal of the EFAI. At the same time, product species are more strongly adsorbed on the extracted catalyst. As a result, a small increase in the rate of catalyst decay was noted on the extracted catalyst. The increase in initial rate of conversion is therefore negated at longer times on stream by the increase in the rate of catalyst decay. Selectivity data show that bimolecular reactions involving carbenium ions and feed molecules are dominant on both catalysts. No significant change in product distribution was observed upon removal of the extraframework aluminum. We conclude that the extraction of the EFAI increases the number of available sites without changing their acidity distribution. Such behaviour would be expected if the role of the EFAI is one involving purely mechanical blocking of feed accessibility to a random selection of active sites.