Potassium-modified ZrO2 base catalysts were prepared by wet impregnation of hydrous zirconia and anhydrous zirconia with potassium compounds and calcined at 600 degreesC in air. The properties of the catalysts were characterized and compared. Catalytic activities were examined for the vapor-phase double-bond isomerization of 1-butene at 150 degreesC and the liquid-phase Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone at room temperature. The properties of the modified ZrO2 i.e., basic site strength, were greatly affected by the ZrO2 precursors and the potassium modifiers. High yields of 2-butene with relatively high cis/trans ratios were found. Leaching of the potassium-modified ZrO2 makes the system less suitable for use in liquid-phase reactions.