Book excerpt: The concept that the disproportionation reactions of tetravalent and pentavalent plutonium ions are special cases of the general plutonium equilibrium problem is illustrated with the estimation of the distribution of plutonium valence states when tetravalent and pentavalent plutonium ions are allowed to disproportionate in the presence of excess hexavalent plutonium. Sources of errors in disproportionation stoichiometry coefficients are described, and a table of relative errors is presented. A computer program suitable for estimating plutonium valence state distribution for any initial plutonium oxidation number is included. Application of the program to the study of the autoreduction of solutions of hexavalent 238Pu and 239Pu is suggested.