Mono- and disubstitution of HCo(CO)4 with tertiary phosphines and phosphites was studied by IR and 1H-NMR spectroscopy. These substitutions proceed through phosphonium tetracarbonylcobaltate intermediates, leading to a mixt. of isomers. The crystal and mol. structure of trans-HCo(CO)3[P(O-o-C6H4Ph)3] was detd. by x-ray diffraction.