The transient recovery of the high-frequency capacitance of a deeply depleted MOS capacitor initially biased in the inversion region is analysed using a generation-recombination model which assumes a dominant discrete trap in the bulk and a uniform density of traps distributed in energy at the surface. This model enables the semi-conductor to be characterized in terms of its minority carrier recombination lifetime in the neutral bulk, its dominant bulk trap energy and its surface recombination velocity at flat-band conditions, all of which are time-independent parameters. The analysis is then used to extract these parameters from capacitance against time recovery curves measured on experimental silicon MOS structures. The difficulties associated with the deep depletion method of measuring lifetime and surface recombination velocity are pointed out, and a new technique involving the effect of optical injection on the MOS deep depletion recovery is described and used to determine the bulk lifetime independently of surface effects.