The on-off intermittent behavior of emission from single CdSe/CdS core/shell nanocrystals was investigated as a function of temperature and excitation intensity. Off times were found to be independent of excitation power and the temperature dependence reveals substantial reduction in the number of on-off cycles prior to final particle darkening at low temperatures. On times are found to vary linearly with excitation intensity over a broad range and the turn off rate shows activated Arrhenius behavior down to T=50 K. These observations are consistent with a darkening mechanism that is a combination of Auger photoionization and thermal trapping of charge. The inhomogeneity of various possible tray sites is discussed. A thermally activated neutralization process is required for the particle to return to the on state. The influence of shell composition on intermittency is compared for CdS and ZnS [M. Nirmal et al., Nature 383, 802 (1996)].