AbstractA novel trans‐platinum(II) complex bearing one dimethylamine (dma) and one methylamine (ma) ligand, namely trans‐[PtCl2(dma)(ma)], recently synthesised and characterised in our laboratory, displayed relevant antiproliferative properties in vitro, being more active than the parent complex, trans‐[PtCl2(dma)(ipa)], which has isopropylamine (ipa) in place of methylamine. We have analysed comparatively the solution behaviour of these two complexes under various experimental conditions, and investigated their reactivity with horse heart cytochrome c by mass spectrometry, inductively coupled plasma–optical emission spectroscopy (ICP‐OES), 2D [1H,15N],[1H,13C] HSQC and [1H,1H] NOESY NMR. Some important changes that occurred in the [1H,13C] HSQC NMR spectrum of cytochrome c treated with trans‐[PtCl2(dma)(ma)] in water, after two days’ incubation, most probably arose from direct platinum coordination to the protein side chain; this was proved conclusively by [1H,1H] NOESY NMR and [1H,15N] HSQC NMR measurements. Met65 was identified as the primary Pt binding site on cytochrome c. Electrospray mass spectrometry (ESIMS) results provided evidence for extensive platinum–protein adduct formation. A fragment of the [Pt(amine)(amine′)] type was established to be primarily responsible for protein metalation. ICP‐OES analysis revealed that these trans‐platinum(II) complexes bind preferentially to the serum proteins albumin and transferrin rather than to calf thymus DNA. Pt binding to DNA was found to be far lower than in the case of cisplatin. The implications of the results for the mechanism of action of novel cytotoxic trans‐platinum complexes are discussed.