The deactivation of an ultrastable Y-zeolite during cracking of n-octane, isooctane and 1-octene was studied in a fixed-bed reactor. The relative reactivities of the three reactants in increasing order were n-octane < isooctane < 1- octene. At high residence times, complete conversion was achieved followed by a relatively slow decline that, in fact, presented a falsified deactivation picture. On the other hand, a surprising olefin yield increase with time onstream was observed, i.e., isobutene during isooctane cracking and octene isomers in the case of 1- octene. The isobutene yield increase was assigned to preferential deactivation of strong acid sites that allowed primary products to desorb into the gas phase, increasing their apparent yield rather than undergo secondary reactions. Similarly, 1-octene isomers showed a yield increase with time onstream due to stronger deactivation of cracking reactions than isomerization.