The spray pyrolysis technique is used to generate smooth, spherical zirconia particles, suitable as probes in direct force measurements using the atomic force microscope (AFM). Both monoclinic and tetragonal crystal structure spheres can be produced. The zirconia spheres are characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and AFM imaging. AFM measurements of the force between the zirconia probe spheres and a mica surface enable the determination of the zirconia sphere surface potential and hence the isoelectric point of the zirconia surface. A value of ca. 3.3 is found for both the monoclinic and tetragonal crystal structure spheres. The difference between this value and the higher values previously reported for colloidal systems is provisionally explained as being due to the influence, in the colloidal systems, of specific adsorption of cationic zirconium hydroxo species.