Iron in the +5 oxidation state was generated by the reduction of aqueous potassium ferrate(VI), K2FeO4, using pulse radiolytically generated free radicals. The predominant mode of decay is first-order, in the pH range 3.6 to 7, with the rate constant decreasing from 7 x 10(4) s(-1) to about 100 s(-1). The instability of K2FeO4 under the conditions used required the use of the premix pulse radiolysis technique in which the pulse is delivered 100-200 ms after mixing of K2FeO4 solution with buffers. The rate law and the observation of small pH-dependent spectral shifts indicate that ferrate(V) exists in at least three protonated forms that, formulated as tetrahedral species, form the following equilibria : H3FeO4 half arrow right over half arrow left H2FeO4- + H+, 5.5 less than or equal to pK(1) less than or equal to 6.5; H2FeO4- half arrow right over half arrow left HFeO42- + H+, pK(2) approximate to 7.2; HFeO42- half arrow right over half arrow left FeO43- + H+, pK(3) = 10.1. The rate-limiting process in the first-order decay is the aquation of a tetrahedral ferrate(V) species into a species which is probably six-coordinate. The subsequent first order decay of the octahedral species is so fast that a second-order decay mode for H3FeO4 and H2FeO4- is not observed. The species present in alkaline solution, FeO43- and HFeO42-, aquate very slowly (less than or equal to 10 s(-1)) decay predominatly by a second-order process.