Emission lifetimes of band edge and deep trap states of CdS nanoparticles with different surface capping were measured. For unpassivated nanoparticles with low fluorescence, the emission is dominated by deep trap states and the decay (5 ns) is independent of excitation intensity. For surface passivated nanoparticles with strong luminescence, the emission is dominated by band edge states with a fast and slow decay (a few ns and 50 ns). The slow decay is independent of intensity and the fast component is strongly dependent on intensity. The results support the model of exciton-exciton annihilation upon trap state saturation at high intensities.