The surface Lewis acidity of pure monoclinic ZrO{sub 2} activated at three different temperatures (670, 870, and 1,070 K) has been studied from a quantitative and energetic point of view by means of adsorption of CO at room temperature, coupling IR spectroscopy, and adsorption microcalorimetry. Two different CO adspecies are formed at the surface, whose spectral position, heat of adsorption, and relative population depend on the extent of surface dehydroxylation: a high-frequency (CO){sub H} species (2,192-2,195 cm{sup {minus}1}, 65-73 kJ mol{sup {minus}1}, 0.18-0.24 CO molecule nm{sup {minus}2} on ZrO{sub 2} outgassed at 670 and 870 K, respectively) and a low-frequency (CO){sub L} species (2,184-2,187 cm{sup {minus}1}, 44-50 kJ mol{sup {minus}1}, 0.50-0.80 CO molecule nm{sup {minus}2} on ZrO{sub 2} outgassed at 670 and 870 K, respectively). At 1,070 K, the onset of the sintering causes a marked decrease in the population of the high-frequency species but does not modify the acidic strength of the two sites.