The reactions HCO (DCO) + O-2 have been measured by the laser photolysis/CW laser-induced fluorescence (LIF) method from 296 to 673 K, probing the (B-2 A ' - X-2 A ') HCO (DCO) system. The HCO (DCO) + O-2 rate coefficients are 5.63 +/- 0.31 and 5.61 +/- 0.23 x 10(-12) cm(3) molecule(-1) s(-1), respectively, at 296 K; both are nearly independent of temperature between 296 and 673 K. The observed deuterium kinetic isotope effect is within the error estimate of previous measurements but is significantly smaller than recent theoretical predictions.