The nature, concentration, and location of cationic lanthanum species in faujasite-type zeolites (zeolite X, Y and ultrastabilized Y) have been studied in order to understand better their role in hydrocarbon activation. By combining detailed physicochemical characterization and DFT calculations, we demonstrated that lanthanum cations are predominantly stabilized within sodalite cages in the form of multinuclear OH-bridged lanthanum clusters or as monomeric La3+ at the SI sites. In high-silica faujasites (Si/Al = 4), monomeric [La(OH)]2+ and [La(OH)2]+ species were only found in low concentrations at SII sites in the supercages, whereas the dominant part of La3+ is present as multinuclear OH-bridged cationic aggregates within the sodalite cages. Similarly, in the low-silica (Si/Al = 1.2) La–X zeolite, the SI' sites were populated by hydroxylated La species in the form of OH-bridged bi- and trinuclear clusters. In this case, the substantial repulsion between the La3+ cations confined within the small sodalite cages induces the migration of La3+ cations into the supercage SII sites. The uniquely strong polarization of hydrocarbon molecules sorbed in La–X zeolites is caused solely by the interaction with the accessible isolated La3+ cations.