In many instances the quality of the surface in ZnO nanoscale systems is a key performance-defining parameter. The surface itself could be a very significant source of lattice defects as well as contaminating impurities, and this influence may extend into the sub-surface vicinity. In our work, key element of the surface analysis is the surface photovoltage (SPV) spectroscopy known for its advantages, such as: identification of conduction vs. valence band nature of the defect-related transitions and the defect level positions within the band gap, ability to measure relatively low densities of surface defects as well as their cross sections. Additional information can be obtained from the SPV transient measurements. In our system, SPV characterization is run in high vacuum, complemented by in situ remote plasma treatment. This combination of surface-sensitive and surface-specific tools is well-suited for studying surface properties with a high degree of reliability since there is no exposure to common air contaminants between processing and characterization cycles. We employed O/He remote plasma treatments of ZnO nanocrystalline surfaces. In situ SPV spectra and transient measurements of the as-received and processed samples revealed, on the one hand, a number of common spectral features in different ZnO nanopowder specimens, and, on the other hand, a noticeable plasma-driven changes in the surface defect properties, as well as in the overall electronic and optical surface characteristics.