The overall rate constant for the radical-radical reaction C2H5 + HO2 -> products has been determined at room temperature by means of time-resolved mass spectrometry using a laser photolysis/flow reactor combination. Excimer laser photolysis of gas mixtures containing ethane, hydrogen peroxide, and oxalyl chloride was employed to generate controlled concentrations of C2H5 and HO2 radicals by the fast H abstraction reactions of the primary radicals Cl and OH with C2H6 and H2O2, respectively. By careful adjustments of the radical precursor concentrations, the title reaction could be measured under almost pseudo-first-order conditions with the concentration of HO2 in large excess over that of C2H5. From detailed numerical simulations of the measured concentration-time profiles Of C2H5 and HO2, the overall rate constant for the reaction was found to be k(1)(293 K) = (3.1 +/- 1.0) x 10(13) cm(3) mol(-1) s(-1). C2H5O could be confirmed as a direct reaction product.