AbstractThe flash photolysis of aqueous solutions of rhodizonic and croconic acids has been studied in the presence and absence of electron acceptors. No transient absorption which could be identified with an excited state was observed with either anion. The rate of recovery of the ground state in the absence of additives was a first‐order process with both acids and gave rate constants for deactivation of the excited state,kD, of 2.4 × 105s−1for rhodizonate and 2.8 × 105s−1for croconate. With croconate dianion in the presence of three acceptors, 4‐nitrobenzylbromide, methylviologen, and biacetyl, a transient absorption was detected, with a maximum absorbance at 500 nm, and was tentatively identified with the monoanion radical, formed following electron transfer to the acceptor. From the rate of growth of the transient, rate constants for the rate of electron transfer to the acceptor were measured as follows: 4‐nitrobenzylbromide: 2.8 × 109M−1s−1; methyl viologen: 3.7 × 1010M−1s−1; and biacetyl: 2.0 × 108M−1s−1. The significance of the measurements is discussed in relation to the mechanism proposed for the photochemical reactions of these dianions.