The equilibrium constant K-d of disproportionation 2[PtCl4](2-) = [PtCl6](2-) + 2Cl(-) + Pt was studied as a function of state of metal platinum. For the reaction of compact platinum in 1 M HCl, values of 20.0 +/- 0.8 and 12.2 +/- 0.6 were found for K-d at 130 and 180 degreesC. Experimental evidence shows that the equilibrium constant of platinum(II) disproportionation is a function of the extent of platinum metal dispersion. X-ray diffraction was employed to study variations in domain size and microstrain in the course of the straightforward and back reactions.