The mechanism of the gas-phase OH-initiated oxidation of glycolaldehyde (HOCH2CHO) was studied in the 233-296 K temperature range using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. In the presence of O-2, formaldehyde, CO2, formic acid, and glyoxal were observed at room temperature with the yields of 80, 34, 18, and 14%, respectively. Decrease of temperature to 233 K led to significant changes in the yields of the stable products: those of formaldehyde and glyoxal decreased to 50 and 4%, respectively, whereas that of formic acid increased to 52%. It was also found that the OH + glycolaldehyde + O-2 reaction proceeds with considerable reformation of OH radicals (by 25% at 296 K). The observed product yields are explained by a mechanism including formation of short-lived intermediate adducts of the primary radicals with O-2. The implication of the obtained results for the HOx budget in the upper troposphere is discussed.