A series of p-substituted benzoylsilanes have been synthesized and their infrared and ultraviolet spectra determined. Correlations of both the infrared carbonyl stretching frequency and the n–π* ultraviolet absorption with Hammett substituent constants have been found. Benzoylsilanes are generally unstable in ethanol undergoing light-catalyzed decomposition. Benzoyldiphenylphosphine was found to give spectra similar to those of benzoylsilanes.