Co/NaY, Pt/NaY, Ru/NaY, Ru-Co/NaY and Pt-Co/NaY samples were pretreated in different ways and their Bronsted and Lewis acidities were measured by means of pyridine adsorption and by double-bond isomerization of 1-butene. It is established that during nitrogen/hydrogen pretreatment small metal or bimetallic particles are formed and the system possesses Bronsted and Lewis acidity, whereas by pretreatment in oxygen/hydrogen only Lewis acidity is developed (large metal particles segregated to the zeolite surface). The difference is due to the formation of proton-metal adducts inside the zeolite supercage when nitrogen/hydrogen pretreatment is applied and this is ceased when oxygen/hydrogen pretreatment is used. This is fully supported by the double-bond isomerization of 1-butene to cis- and trans 2-butenes.