Time-resolved luminescence measurements are performed on high-quality ZnO nanocrystals in a quantum confinement regime prepared by fast cooling and succeeding surface passivation. A very long luminescence lifetime of ∼0.1 μs, which is indicative of the existence of a dark exciton state, is observed. The temperature dependence of the lifetime and the luminescence peak energy is successfully accounted for by a thermal equilibrium model of excitonic dark and bright states. © 2011 American Physical Society.