Among all existing complex organic molecules, glycolaldehyde HOCH2CHO and ethyleneglycol HOCH2 CH2OH are two of the largest detected molecules in the interstellar medium. Weinvestigate both experimentally and theoretically the low-temperature reaction pathways lead-ing to glycolaldehyde and ethylene glycol in interstellar grains. Using infrared spectroscopy,mass spectroscopy and quantum calculations, we investigate formation pathways of glyco-laldehyde and ethylene glycol based on HCO• and •CH2 OH radical–radical recombinations.We also show that •CH2 OH is the main intermediate radical species in the H2 CO to CH3 OHhydrogenation processes. We then discuss astrophysical implications of the chemical pathwaywe propose on the observed gas-phase ethylene glycol and glycolaldehyde.