The chloride ligand trans to PEt₂Ph in mer-[IrCl₃(PEt₂Ph)₃] has been found to be labile and readily displaced by cyanide ion in methanol to give trans,mer-[IrCl₂(CN)(PEt₂Ph)₃] as the only isomer. The trans,mer isomer may also be synthesized by treating trans,mer-[IrCl₂(H₂O)(PEt₂Ph)₃][ClO₄] with cyanide ions. Irradiation of this isomer in dichloromethane with visible white light led initially to a mixture of the trans,mer isomer with the fac and cis,mer isomers, while extended irradiation gave almost exclusively the cis,mer isomer with none of the fac or trans,mer forms remaining. This contrasts with the visible irradiation of mer-[IrCl₃(PEt₂Ph)₃] which gave exclusively the fac isomer. Reactions were followed by ³¹P-{¹H} and ¹³C-{¹H} NMR spectroscopy of ¹³CN-enriched samples and by Fourier-transform IR spectroscopy at around 2000 cm^–1. The crystal structures of the trans,mer and cis,mer isomers of [IrCl₂(CN)(PEt₂Ph)₃] have been determined. During the photolysis a cyano-bridged species derived from the fac isomer, probably fac,fac-[(PhEt₂P)₃Cl₂Ir(µ-CN)IrCl(CN)(PEt₂Ph)₃]Cl, is observed as an intermediate but was not isolated. This salt disappears on extended irradiation. Addition of an excess of [N(PPh₃)₂]Cl totally suppressed the formation of the µ-CN species, consistent with the formula given, while only slightly affecting the rate of conversion of the trans,mer into the fac and cis,mer isomers on irradiation.