It was observed that in toluene solution, excess 1-methyl-imidazole (Melm) reduces dichloro 5, 10, 15, 20 tetraphenyl-porphinatoplatinum (IV) (PtTPPCl2) to tetraphenylporphinato-platinum(II) (PtTPP) autocatalytically (Fig.1). There was no Pt(III) intermediate or 1-methylimidazole radical detected by Electron Resonance Spectroscopy (ESR) at 25 degrees C. The reaction was found to be autocatalyzed by the PtTPP product when the 1-methylimidazole to PtTPPCl2 concentration ratio is about 10(4) or more. From the data obtained it is reasonable to suppose that aggregation of the PtTPP product with PtTPPCl2 plays an important role in this reduction process.