Biacetyl (butane-2,3-dione) CH3C(O)C(O)CH3 has been photolysed (λ= 390–470 nm) at atmospheric pressure in Ar : O2= 80 : 20 (mol/mol) mixtures between 263 and 333 K under steady-state conditions. Reaction products were analysed by matrix-isolation FTIR spectroscopy combined with a molecular-beam sampling technique. Observed reaction products were CO2, CO, HCHO, HCOOH, CH3C(O)OH, CH3C(O)OOH, CH3OH, H2O2 and O3. Based on an assumed mechanism for the reaction, quantitative product data were used to determine the temperature dependence of the branching ratios β10 and β11 of the following two sets of CH3C(O)O2 radical reactions: CH3C(O)O2+ HO2→ CH3C(O)OOH + O2(10a), CH3C(O)O2+ HO2→ CH3C(O)OH + O3(10b), β10=k10a/k10b= 330 × exp(–1430/T) and CH3C(O)O2+ CH3O2→ CH3C(O)O + CH3O + O2(11a), CH3C(O)O2+ CH3O2→ CH3C(O)OH + HCHO + O2(11b), β11=k11a/k11b= 2.2 × 106 exp(– 3870/T)