A minor CH(a(4) Sigma(-) and/or X(2) Pi) plus CO2 producing channel of the reaction between ketenyl radicals and oxygen atoms was identified as the probable source of CH(a(4) Sigma(-)) radicals in room temperature C2H2/O systems : HCCO + O --> CH(a(4) Sigma(-) and/or X(2) Pi) + CO2 (r2b). Using the discharge-flow/molecular beam sampling mass spectrometry technique, reaction r2b was found to be the dominant CO2 source in such systems. Hence. the observed CO2 production could be used as a tracer to quantify CH formation by the reaction between HCCO and O. The only other critical parameter, the rate constant k(1a) of the primary reaction channel C2H2 + O --> HCCO + H, is well established. Contributions from other, minor CO2 sources were accounted for by kinetic modeling. In this way, the CO2, plus CH yield of reaction r2 at T = 290 K was found to be k(2b)/k(2) = 0.062 +/- 0.024 (95% overall confidence region); using the known k(2) = 1.3 x 10(-10) cm(3) molecule(-1) s(-1), a value of k(2b) - (8.0 +/- 3.1) X 10(-12) cm(3) molecule(-1) s(-1) is derived. Although an experimental proof far CH(a(4) Sigma(-)) formation in the newly identified reaction r2b was not obtained, an examination of the different reaction pathways on the HCCO + O potential energy surfaces, calculated at the QCISD(T)/6-311++G-(d,p)//UMP2/6-31G(d,p)+ZPE level of molecular orbital theory, leads one to expect that a sizeable fraction of the methylidyne radicals arises in the quarter a(4) Sigma(-) state. Each of the characterized CH-producing pathways of HCCO + O should become faster at elevated temperatures; therefore, it is suggested that CH(a(4) Sigma(-) and/or X(2) Pi) + CO2 production by HCCO + O can be an important process in hydrocarbon flames.