Reactivity of phenyl radical towards propionaldehyde and butyraldehyde were investigated by the CVT/SCT/ISPE method using the energetics computed at the CCSD(T)//B3LYP/cc-pVTZ level of theory over the temperature range of 200-2000 K. The 298 K rate coefficients (k × 1014 cm3 molecule-1 sec-1) for two aldehydes were computed to be 7.7 and 15.2. The branching ratios explained the contribution of various H-abstraction reaction channels to the global reactivity. The degradative mechanism explained the formation of acids, aldehydes and peroxy nitrates was proposed. The reactivity of these aldehydes towards phenyl radical was also compared to their reactivity towards other atmospheric oxidants.