Phosphine isocyanide complexes cis-[PtCl ₂(CNMes)(P)] with mesitylisocyanide and phoshine ligands were synthesized in yields of 92‒98%. The products were characterized by mass spectrometry, IR and ¹H NMR, COSY, NOESY, HSQC, and HMBC spectroscopy, and X-ray diffraction analysis. The solid-state and solution structures of the complexes and their stability in isomerization and ligand disproportionation reactions were studied.