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cis-4-Hydroperoxycyclophosphamide (5) undergoes facile reaction with aqueous phosphate or Tris buffers at pH 7-8 and 30 degrees C. The kinetics of 5 are complex, and the trans-4-hydroperoxy isomer 6 is produced and subsequently disappears over the course of the reaction. Addition of hydrogen peroxide to the reaction mixture retards the disappearance rate of 5 and increases the amount of 6 generated. Rate constants for the reversible disappearance of 5 and appearance of 6 and 4-hydroxycyclophosphamide (2) have been determined by nonlinear least-squares methods. The reaction is catalyzed by hydroxide ion, Tris free base, and HPO4 2-, with catalytic constants of 0.032 min-1 (pH 8.0), 0.052, and 0.115 M-1 min-1, respectively. The major product in the presence of Tris is the oxazolidine 8b arising from the addition of Tris to aldophosphamide, not 2 as assumed previously. These results are consistent with a mechanism involving general-base-catalyzed elimination to produce iminocyclophosphamide 7 as a transient intermediate; the imine can react with the hydrogen peroxide evolved in the reaction to give 5 and 6, with water to give 2, or, in general, by addition of a nucleophile to C-4. The significance of these findings with respect to other 4-substituted cyclophosphamides is discussed.