NaH-USY, H-USY, and Cu(II)-exchanged NaH-USY samples prepared were studied for ion-exchange mass balance, framework aluminum microenvironments (FAI-MEs), and number and strength of acid sites (AS) generated by the increase of the Cu(Il)-exchange after thermal treatment (500 degrees C) and lost under hydrothermal treatment conditions in the presence and in the absence of V species. From the data obtained, conclusions were drawn that (a) the nature and number of Cu(II) species exchanged enter FAI-MEs which can compensate their charge, (b) the strength and number of generated AS in CuNaH-USY samples after thermal treatment is related to the nature and number of Cu(Il) species accommodated in the corresponding FAI-MEs, (c) the AS lost during the hydrothermal treatment depend on the nature of counterion involved, and (d) in the presence of V species, an extra loss, restricted to strong AS, takes place. Differences of lost AS and structure - stability during hydrothermal treatment in the presence and absence of V species give evidence of V catalytic action and support the implication of the nature of counterion species for the degree of structure breakdown. These findings lead to the proposed reaction pathway for the chemistry of V detrimental effect.