(High resolution) TEM and FTIR spectroscopy of in-situ CO adsorption at 300and ∼ 78 K have been used to characterize particle morphology and surface Lewis acidity of three preparations of tetragonal ZrO2 stabilized with Y2O3. Depending on preparation, the starting materials are in the form of individual coin-shaped platelets, or of huge aggregates of small particles. In either case, few low-index crystal planes are most frequently exposed, and the particle edges are irregular. The adsorption of CO indicates that several families of coordinating centres are present, among which some highly energetic Lewis sites (νCO > 2190 cm-1) that are most likely localized in defective configurations (e.g., on particle edges and steps). Thermal treatments at T as high as ∼ 1200 K cause the onset of the sintering process, and bring about gradual modifications of particles size, shape and termination. Consistently, CO adsorption indicates that the most energetic sites (high νCO) are selectively eliminated, whereas new families of less and less energetic sites are produced. The partial segregation of different phases depends on preparation parameters.