The response of m.o.s. devices to a fast linear voltage ramp, when under illumination, is analysed. The presence of traps in the semiconductor bulk is taken into account, and it is found that bulk generation and photogeneration are similar, but not identical, effects. The devices exhibit a photoresponse up to a maximum value of light intensity, when saturation occurs, and no further light sensitivity is observed. Experimental data is presented for conditions when bulk generation is significant and insignificant, and good agreement is found in each case between theory and experiment.