It has recently been discovered that carbonyl compounds can undergo UV-induced isomerization to their enol counterparts under atmospheric conditions. This study investigates the photoisomerization of glycolaldehyde (HOCH2CHO) to 1,2-ethenediol (HOCH=CHOH) and the subsequent (OH)-O-center dot-initiated oxidation chemistry of the latter using quantum chemical calculations and stochastic master equation simulations. The keto enol tautomerization of glycolaldehyde to 1,2-ethenediol is associated with a barrier of 66 kcal mol(-1) and involves a double-hydrogen shift mechanism to give the lower-energy Z isomer. This barrier lies below the energy of the UV/vis absorption band of glycolaldehyde and is also considerably below the energy of the products resulting from photolytic decomposition. The subsequent atmospheric oxidation of 1,2-ethenediol by (OH)-O-center dot is initiated by addition of the radical to the pi system to give the (CH)-C-center dot(OH)CH(OH)(2) radical, which is subsequently trapped by O-2 to form the peroxyl radical (O2CH)-O-center dot(OH)CH(OH)(2). According to kinetic simulations, collisional deactivation of the latter is negligible and cannot compete with rapid fragmentation reactions, which lead to (i) formation of glyoxal hydrate [CH(OH)(2)CHO] and HO2 center dot through an alpha-hydroxyl mechanism (96%) and (ii) two molecules of formic acid with release of (OH)-O-center dot through a beta-hydroxyl pathway (4%). Phenomenological rate coefficients for these two reaction channels were obtained for use in atmospheric chemical modeling. At tropospheric (OH)-O-center dot concentrations, the lifetime of 1,2-ethenediol toward reaction with (OH)-O-center dot is calculated to be 68 h.