The effects of both Si/Al ratio and preparation method (ion exchange vs. impregnation) on the physico-chemical properties of potassium modified Y zeolites were studied in detail. The samples were used as basic catalysts for aldol condensation of furfural and acetone. A relationship between physico-chemical properties and catalytic performance of potassium-containing Y zeolites was established using XRD, N-2 physisorption, IR spectroscopy of adsorbed CO2 and CO2-TPD. Neither ion-exchange nor impregnation with KNO3 removed completely all acid sites in Y (Si/Al = 2.5). Moreover, K2O clusters were not formed under thermal activation of the samples prepared from the parent H-Y(2.5). As a consequence, both ion-exchanged and impregnated K-Y(2.5) possessed low activity in aldol condensation, but they had high ability to dehydrate reaction products. In contrast, thermal activation of USY zeolites (Si/Al = 15 and 40) impregnated with potassium resulted in the formation of K2O species which, according to CO2-IR spectroscopy and CO2-TPD, were strong basic sites. It was concluded that the lower amount of proton sites available for an ion-exchange together with the larger mesopore volume and more defective crystalline framework promoted the accumulation of KNO3 in the USY zeolites during the impregnation step. Subsequently, K2O species were formed by thermal treatment. Impregnated and calcined USY zeolites possessed a superior catalytic activity in comparison with HY (Si/Al = 2.5). The presence of K2O species as strong basic sites in K-impregnated USY zeolites was favorable for both the high activity of the catalysts in aldol condensation and the occurrence of the second condensation step.