ZnO nanoparticle inception within mordenite and ZSM-5 zeolites, of varied Si/Al ratios, was achieved via the exchange of zeolite cations with Zn2+, followed by NaOH leaching, and thermal treatment at 550 °C under O2. ZnO particles were observed by electron microscopy; further characterization included X-ray diffraction, thermogravimetric analysis, and N2 sorption. NaOH addition caused a partial desilication of mordenite and ZSM-5 zeolites as shown by a decreased Si/Al ratio. The bulk and surface Zn and Al concentrations were measured by ICP-AE spectrometry and XPS, respectively. N2 sorption isotherms on ZnO/zeolite specimens indicated the existence of micropores and mesopores; the amounts of them in mordenite and ZSM-5 zeolites were determined from adsorption data at low pressures. The progressive growth of ZnO nanoparticles induced some micropore widening as well as the inception of a slit-like intrazeolite mesoporosity due to the desilication of the substrate during the NaOH leaching process. The final zeolite products are potentially useful as they are endowed with a twofold micro-mesoporosity.