Book excerpt: The rates of polymerization and disproportionation of Pu(IV) have been studied using low concentrations: (1.7 - 10) x 10−M Pu, (0.8 - 12) x 10−M HCl and 0.0iM ionic strength. Osmium(II) complexes such as the tris-4,4'-2,2'-bipyridine complex were found to react rapidly with Pu(IV) but very slowly, if at all, with Pu(IV) polymer, Pu(III), or Pu(V). Thus, it is possible to determine unreacted Pu(IV) in the presence of rection products by using Os(II) complexes. Disproportionation reaction products, Pu(III) and Pu(V), were determined using their reactions with Ce(IV) sulfate. We find -d[Pu(IV)]/dt = k'[Pu(IV)]2 at constant pH. Log k' varies from about 4.25 at pH 3 to about 7.0 at pH 4.1 (units for k' are M−1min−1). The [H] dependence varies from about -2 to -3 over the pH range studied. The measured rate is the sum of those for polymerization and disproportionation; the latter reaction amounts to about 75% of the total at pH 3 and 20% at pH 4. The second-order rate constants for disproportionation are very much larger than expected on the basis of extrapolation from 0.2 to 1.0M HC104 solutions. The products of the reaction do not affect the rate, but U(VI), aged Pu(IV) polymer, and CO2 increase the rate.