Measurements of quantum yields of phosphorescence are reported for a series of aromatic ketones and aldehydes in a glass at 77 K. Values for the radiative and radiationless rate-constants are calculated from the associated lifetimes. For molecules with 3nπ* as the lowest excited state a correlation between the spin-orbit electronic matrix elements for the radiative and radiationless processes is observed. The radiative and radiationless matrix elements are due to mixing with an excited ππ* state and the ground state respectively, and these are discussed in terms of the one-centre integrals on the oxygen atom. Quantitative estimates of the radiationless rate-constant indicate that the dominating vibration in the Franck-Condon factor is the carbonyl stretch. For molecules with 3ππ* as the lowest excited state, both radiative and radiationless processes occur through spin-orbit mixing with the 1nπ* state. As the 3ππ*, 1nπ* energy gap increases the phosphorescence characteristics approach those of aromatic hydrocarbons. Radiationless processes in fluorenone, showing ππ* fluorescence but little phosphorescence are also discussed.