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NaY zeolites measuring 63 and 193 nm (average crystallite size) have been synthesized by dosing different amounts of seeds in the crystallization step. Evaluation of catalytic activity of both zeolites after ion exchange with rare-earth ions and calcination has been carried out with two reactants of different molecular size: n-heptane and 1,3,5-triisopropylbenzene. No influence of the crystallite size (and therefore of the external surface area) was observed for the cracking of n-heptane, as expected previously. However, when the cracking of 1,3,5-triisopropylbenzene was performed, smaller-crystallite zeolite presented much higher conversions, as an indication that the reaction takes place at the outer surface of the zeolite. In fact, rates of reaction per external atoms of aluminum per unit cell, measured by XPS, are coincident, regardless of the crystallite size, confirming the assumption established above.