Cavity ring-down absorption spectroscopy was used to measure k(HCO + O-2) = (5.9 +/- 0.5) x 10(-12), k(HCO + NO) =(1.9 +/-0.2) x 10(-11), and k(HCO + Cl-2) = (7.6 +/- 0.7) x 10(-12) cm(3) molecule(-1) s(-1) in 4-10 Torr of N-2 diluent at 295 K. FTIR/smog-chamber techniques were used to measure the following rate constant ratios in 15-750 Torr of N-2 diluent at 295K: k(HCO + O-2)/k(HCO + Cl-2) = 0.85 +/- 0.02, k(HCO + NO)/k(HCO + Cl-2) = 2.80 +/- 0.10, and k(HCO + NO2)/k(HCO +Cl-2) = 8.45 +/-0.38. Consistent results were obtained from the two different techniques. In 15-700 Torr of N-2 diluent at 295 K the reaction of HCO with Cl-2 proceeds via a single channel giving HC(O)Cl + Cl, reaction of PICO with NO gives CO in a yield indistinguishable from 100%, and reaction of HCO with NO2 gives a 70% yield of CO and a 30% yield of CO2. Ab initio calculations show that the reaction of HCO radicals with Cl-2 proceeds via the formation of the HC(O)Cl-2 complex, which decomposes, rapidly to HC(O)Cl and a Cl atom.