A high-temperature experimental and modeling study of the key reactions occurring in (CO2 + H2S) and (CO + H2S) mixtures is reported. The experiments were conducted under dilute conditions (dilution >98%) in tubular flow reactors over a temperature range of 800-1200 degrees C at pressures of 120-160 kPa. The corresponding gas residence time ranged from 0.5 to 2.0 s. The experimental results showed a negligible amount of COS formed from reactions of CO2 with H2S and with sulfur, contrary to popular belief. Hydrogen sulfide decomposition into hydrogen and sulfur plays a critical role in the formation of CO and COS and can be described by the simple rate expression: (- r(H2S)) = k(1)C(H2S)C(M), where C-M is the molar concentration of the collisional molecule (N-2) and the rate constant k(1) = 1.68 +/- 0.86 x 10(11) exp [(-28940 +/- 840 K)/T] m(3)/(kmols). Further, the reaction of CO with H2S was found to be rapid and equilibrium COS conversions were attained in less than 700 ms at temperatures greater than 1000 degrees C. The COS forming reaction between CO and H2S could be represented by the rate expression (r(COS)) = k(3)C(CO)C(H2S)(0.5), where the rate constant k(3) = 1.59 +/-0.86 x 10(5) exp[(-13340 +/- 930 K)/T] (m(3)/kmol)(0.5)/s. In a (CO2 + H2S) mixture, the hydrogen produced from H2S decomposition reacts with CO2 to form CO. Finally, the rate of CO formation could be described by the following rate expression: (r(CO)) = k(4)C(CO2)C(H2)(0.5), where k(4) = 3.95 +/- 0.35 x 10(10) exp[(-31220 +/- 180 K)/T] (m(3)/kmol)(0.5)/s.