The effects of lanthanum exchange in zeolite Y and zeolite X were studied by Si-29 NMR. The spectra of calcined LaNaY were curve-fitted by two functions derived from the spectrum of pure NaY, by keeping the Si/Al ratios of the two functions constant and similar to NaY. The fraction of the spectra shifted upfield as a result of lanthanum migration was consistent with the percent La exchange and not with the fractional occupancy of the SI’ sites. Curve fitting of Si-29 NMR spectra of LaNaX was more complicated due to a selective ion exchange of cations close to the Si(4Al) sites in zeolite X. However, when a redistribution of intensity between the two functions was allowed, while keeping the overall Si/Al ratio constant, high consistency between the observed and simulated spectra of LaNaX was found. It is evident from this work that the influence of lanthanum is different for the two zeolites. The results from the combination of the selective ion exchange and Si-29 NMR spectra of LaNaX were found to correlate with those from Monte Carlo simulations and may be used as a complement in the study of the local ordering of framework elements.