Aflatoxin B1 (AFB1) is thought to play a large role in human liver cancer in some parts of the world, and the mechanism of genotoxicity is generally considered to involve the DNA adduct formed at the guanyl N7 atom. The exo epoxide, the genotoxic isomer formed by human cytochrome P450 3A4, has been known to be very unstable in H2O (t 1/2 < 10 s). The rates of hydrolysis of AFB1 exo epoxide have been determined as a function of pH using stopped flow kinetics. The spontaneous reaction with solvent is faster than previously suspected, with a t 1/2 ∼1 s when either absorbance or fluorescence kinetic traces are measured at ambient temperature. An acid-catalyzed reaction with a bimolecular rate constant of 2 × 103 M-1 s-1 is operative below about pH 5 and elevates the rate. The hydrolysis product, AFB diol, reversibly converts to a furofuran-ring-opened oxyanionic AFB1 α-hydroxydialdehyde (AFB dialdehyde) under slightly basic conditions. When AFB diol was treated with base in DMSO, conversion to AFB dialdehyde was accompanied by dehydration of the remaining alcohol group. In aqueous solution, the AFB diol:AFB dialdehyde equilbrium has been characterized and the pK a (8.2) is much higher than previously suggested; the base-catalyzed bimolecular rate of 2.3 × 103 M-1 s-1 results in a very slow rate of conversion at physiological pH. Multiphasic conversion of AFB dialdehyde back to AFB diol has a slow rate-limiting step at 0.01 s-1 and is nearly quantitative. The dehydrated AFB dialdehyde formed in DMSO did not form a detectable ring closed derivative.