Nonstoichiometric ZnO with an excess of Zn atoms (ZnO:Zn) has a long history of use as a green/monochrome phosphor in electron-excited vacuum fluorescent and field emission displays. The advent of ultraviolet lasers and light emitting diodes presents the possibility of photoexciting the highly efficient, defect-related green emission in ZnO:Zn. Here we study experimentally the time-integrated quantum efficiency and the time-resolved photoluminescence decays of both near band edge and defect emissions in unannealed (ZnO) and annealed (ZnO:Zn) nanoparticles under femtosecond excitation. A comparison of results using one-photon excitation (excitation primarily near the particlés surface) versus two-photon excitation (uniform excitation throughout the particlés volume) elucidates how the quantum efficiencies depend on material properties, such as the spatial distributions of radiative and nonradiative defects, and on optical effects, such as reabsorption. © 2009 SPIE.