The hydridodicarbonyliridium(I) complexes of formula lrH(CO)2L2 where L = PPh3, P(C6H4F)3, PEtPh2, and AsPh3 have been prepared by the interaction of solutions of the corresponding chlorides, trans-IrCl(CO)L2, with sodium borohydride in the presence of carbon monoxide. Although the complexes appear to have a single structure in the solid state, i.r. and n.m.r. spectra show that in solutions in organic solvents two isomeric pentaco-ordinate species in equilibrium are formed. The equilibria and the spectra depend not only on temperature but on the nature of the solvent. The form of the n.m.r. spectra requires that the 31P–H coupling constants in the two isomers, which are undergoing rapid intramolecular interconversion, are opposite in sign. The precise nature of the species in solution is discussed.