The time-resolved formation of OH from ethyl + O 2 and propyl + O 2 reactions has been measured by OH laser-induced fluorescence in pulsed-photolytic Cl-initiated oxidation of ethane and propane between 296 and 700 K. The propane oxidation produces more OH at each temperature than does ethane oxidation. Above 600 K, the peak amplitudes of the OH signals from both reactions increase sharply with increasing temperature. Solutions to the time-dependent master equation for the C 2 H 5 + O 2 , i-C 3 H 7 + O 2 , and n-C 3 H 7 + O 2 reactions, employing previously published ab initio characterizations of the stationary points of the systems, have been used to produce temperature-dependent parameterizations that predict the rate constants for formation of all of the products (R + O 2 , RO 2 , QOOH, OH + aldehydes, OH + O-heterocycles, HO 2 + alkene). These parameterizations are utilized in rate equation models to compare to experimental results for HO 2 and OH formation in Cl-initiated ethane and propane oxidation. The models accurately describe the time behavior and amplitude of the HO 2 from both oxidation systems. However, the model underpredicts the amount of OH observed at high temperatures (>600 K) and overpredicts the amount of OH observed at lower temperatures (e600 K).