Abstract Products of substitution reactions of CO by PPH3 and P(OP)3 in Rh(β-diketone)(CO)2 complexes (where β-diketone: acetylacetone, thenoyltifluoroacetone, trifluoracetone, benzoyltrifluoroacetone, naphthoyltrifluoroacetone) were examined by IR and NMR. Reactions with PPh3 produced the compounds containing one CO group, i.e. Rh(β-diketone(CO)(PPh3). In the case of asymmetric β-diketones, two isomers were observed in solution. The presence of free phosphine caused labilization of the coordination sphere of complexes followed by fast exchange between the free and the coordinated phosphine. Reactions with P(OPh)3 produced Rh(β-diketone)-[P(OPh)3]2 or Rh(β-diketone)[P(OPh)3] complexes, depending on the amount of P(OPh)3 used. The NMR results indicate considerable delocalization of the electron density in these compounds.