The (alkoxycarbonyl)cobalt tetracarbonyl BuOC(O)Co(CO)4 (I) was treated with the ligands ER3 (E = P, As, Sb; R = alkyl, aryl, alkoxy, dialkylamino) to give 37 monosubstituted products BuOC(O)Co(CO)3(ER3). Disubstituted derivs. contg. PR3 (R = Et, OMe) ligands were also made from I. The IR spectra of the compds. contg. various PR3 ligands were correlated with the electronic and steric parameters of the P-ligands. The crystal structure of BuOC(O)Co(CO)3PPh3 was detd. by x-ray crystallog.