A new approach is presented to study the ring-cleavage process of benzene, toluene, and p-xylene (BTX). DOAS (differential optical absorption spectroscopy) was used for the simultaneous measurement of the respective ring-retaining products as well as glyoxal (a ring-cleavage product) in a series of experiments at the EUPHORE outdoor simulation chamber, Valencia/Spain. The good time resolution of the DOAS measurements (1-2 min) allowed the primary formation of glyoxal to be separated from any further contributions through additional pathways via reactions of stable intermediate compounds (secondary glyoxal formation). The ring-retaining products and glyoxal were identified as primary products. The primary glyoxal yield was found to be essentially identical to the overall yield of glyoxal formed over the time scale of the experiments. The negligible contribution from secondary glyoxal formation pathways was quantitatively understood for the experimental conditions of this study and was found to be representative for the troposphere. The yield of glyoxal was determined to be 35% +/-10% for benzene and about 5% higher for toluene and p-xylene. For benzene, the yield of hexadienedial was estimated to be less than or equal to 8%. It is concluded that ring-cleavage pathways involving the bicycloalkyl radical are major pathways in the oxidation of monocyclic aromatic hydrocarbons, i.e., BTX The branching ratio for the bicycloalkyl radical intermediate, proposed to form from the reaction of the aromatic-OH adduct with atmospheric oxygen, could be directly identified with the primary glyoxal yield for the benzene system and as a lower limit in the case of toluene and p-xylene. Implications for the chemical behavior of aromatic hydrocarbons in the atmosphere are discussed.