A number of cobalt carbonyl derivatives of the type [L Co(CO)3]2[L = Et3P, Bu3P, PhMe2P, PhEt2P, Ph3P, (MeO)3P, (PhO)3P, Et3As, or Ph3As] have been prepared, and their infrared spectra investigated in the range 225–4000 cm.–1. Absorption bands due to the vibrations of the Co2(CO)6 moiety have been identified and assigned where possible.