The primary quantum yield of ketene decomposition was studied as a function of pressure, temperature, and wavelength. The results show that excited vibronic states are formed which may be degraded before dissociation can occur. The ratio of the rate of collisional degradation to the rate of dissociation decreases from 164 liters/mole at 3340 Å to 16.8 liters/mole at 3130 Å.