In this work we report examples of a preceding structural change in homogeneous redox catalysis. This behavior was encountered in the catalyzed reduction of xanthylideneanthrone, 1, and thioxanthylideneanthrone, 2. The neutral form of each of these compounds exists in two structures, a minor form (B) and a major form (A). However, in the anion radicals, the preference is reversed with B.- predominating over A(.-). Normal redox catalysis would involve the reduced form of the catalyst, Q, reacting in an uphill reaction with A to produce A(.-) and P, the oxidized form of the catalyst. The unfavorable reaction would be driven by the rapid conversion of A(.-) to B.-. In the case of 1 and 2, however, Q instead reacts with the minor form, B, and the extent of catalysis can be used to evaluate the rate and equilibrium parameters for the preceding structural change, the A reversible arrow B reaction. (c) 2006 Elsevier B.V. All rights reserved.