Absolute rate constants are reported for the reactions of aqueous 1,2-diazene with fumaric acid in its three states of protonation, as well as for the reaction with the maleate dianion. The acid hydrolysis of azodiformate was used as the source of diazene, which underwent competitive disproportionation and substrate hydrogenation. In one set of experiments the decay of diazene was monitored optically in a stopped-flow instrument; the rate constants were extracted from the increased decay rates arising from additions of the substrate. In the other set of experiments the product yields were determined by H-1 NMR spectroscopy; rate constants were derived from these yields by use of the known rate of disproportionation of diazene. The rate constants obtained are (1.32 +/- 0.07) x 10(2) M(-1) s(-1) for fumaric acid, (2.4 +/- 0.5) x 10(2) M(-1) s(-1) for the hydrogen fumarate anion, (8.0 +/- 0.5) x 10(2) M(-1) s(-1) for the fumarate dianion, and 94.5 M(-1) s(-1) for the maleate dianion, all at 25 degrees C. Ab initio calculations on the reaction with the fumarate dianion show that these rate constants, although substantially smaller than for the disporportionation of diazene, are nevertheless in agreement with a synchronous concerted pericyclic double hydrogen atom transfer mechanism.