HZSM-5 with SiO2/Al2O3 = 50 into which 0.5 wt% Cu (Cu/ Al similar to 0.25) was introduced after the removal of extralattice Al species by basic or acidic treatment shows no inhibition of the irreversible loss of active, tetrahedrally coordinated Cu2+ sites upon thermal treatment. Removal of the extralattice Al coupled with a complete exchange of all Bronsted groups for Cu2+ cations results in a noticeable stabilization of the reactive square-planar sites. FTIR data confirm the absence of free Bronsted groups in the exchanged samples. Quantitation of "ESR-visible Cu2+" in these exchanged samples, calcined at 500-800 degrees C in air, gives values of 1.8 and 1.65 wt% for base-treated and acid-treated samples, respectively. These values are near the limit of exchange assuming a Cu/Al=1 stoichiometry in this high-silica material. Formation of aggregated copper species is of minor importance, and the majority of Cu2+ remains well isolated and contributes to the ESR signal, It is evident from the comparison of partially exchanged and completely exchanged samples that one part of Cu2+ ions inhibits the irreversible loss of another portion of Cu2+ quite efficiently. Also, a strong stabilizing effect is noted for samples prepared on acid-pretreated zeolite containing 0.5 wt% Cu and additional Mg2+ cations. Here a large part of the copper ions (> 50%) preserves the parent square-planar Cu2+ state even after calcination at 800 degrees C. The effect of La, Ca, or Sr is also observable but less pronounced, Both the acidic pretreatment and the introduction of an appropriate amount of Mg2+ ions contribute to the better stabilization of the most reactive part of Cu2+ cations in CuMgZSM-5. Magnesium ions compete with Cu2+ for the cationic positions replacing first the less reactive, pyramidal Cu2+ cations, An increase in the Mg2+ loading results in a decrease in the number of the reactive cupric sites in samples calcined at 500-550 degrees C but simultaneously improves further the thermal stability of the square-planar Cu2+ ions.