The rate of the reaction 1, HCO + O-2 -> HO2 + CO, has been determined (i) at room temperature using a slow flow reactor setup (20 mbar < p < 500 mbar) and (ii) in the temperature range 739 K < T < 1108 K behind reflected shock waves (0.82 bar < p < 1.84 bar) employing a perturbation approach. Following the 193 run excimer laser photolysis of mixtures of glyoxal in At, concentration-time profiles were measured using frequency modulation (FM) detection of HCO at a wavelength of lambda = 614.752 nm. Observed differences between HCO concentration-time profiles measured with and without O-2 added to the reaction mixtures could be almost exclusively attributed to reaction 1. The determined rate constants, k(1)(295 K) = (3.55 +/- 0.05) x 10(12) cm(3) mol(-1) s(-1), k(1)(739-1108 K) = 3.7 x 10(13) exp(-13 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (Delta log k(1) = +/- 0.16), reveal a slightly positive temperature dependence of reaction 1 at high temperatures. Furthermore, the 193 nn photolysis of glyoxal, (CHO)(2), has been proven to be an efficient HCO source. Besides HCO, photolysis of the precursor also produces H atoms. The ratio of initially generated H atoms and HCO radicals, f = [H](0)/[HCO](0)(total), was found to depend on the total density p. At room temperature, it varies from f = 1.6 at p = 8 x 10(-7) mol cm(-3) to f = 3.0 at p = 2 x 10(-5) mol cm(-3). H atoms are transformed via reaction 4, H + (CHO)(2) - H-2 + HCO + CO, into additional HCO radicals. The rate constants of reaction 4 were determined from unperturbed photolysis experiments to be k(4)(295 K) = (3.6 +/- 0.3) x 10(10) cm(3) mol(-1) s(-1) and k(4)(769-1107 K) = 5.4 x 10(13)exp(-18 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(Delta log k(4) = +/- 0.12).