In this work, the novel reaction mechanism initiated by the fast CH(2 Pi) + C2H2 --> C3H2 + H reaction (r13a) and followed by C3H2 + O --> C2H + HCO (or H + CO) (r25a) was established as the dominant C2H formation pathway in low-pressure acetylene/atomic oxygen flames at 600 K. The C2H2/O/H flames were investigated in an isothermal discharge-flow reactor at a pressure of 2 Torr, with He as bath gas. Concentration vs reaction-time data were obtained by molecular beam sampling and threshold ionization mass spectrometry. The crucial role of CH((2) Pi) was evidenced by CH4-addition experiments on room-temperature C2H2/O/H systems, where CH((2) Pi) and CH2((1)A(1)) are the sole intermediates that react rapidly with CH4. The observed strong reduction of [C3H2], [C2H], and [C4H2] upon CH4 addition could be correlated quantitatively with the known removal of CH(2 Pi) by CH4. The reaction channels r13a and r25a as sources of C3H2 and C2H, respectively, were each established by quasi-steady-state analyses of the pertaining radicals in C2H2/O/H mixtures at 600 K. By a similar method, the observed C3H radicals could be attributed to a minor channel of the CH((2) Pi) + C2H2 reaction (r13) parallel to that producing C3H2. At 600 K and 2 Torr, the following approximate product yields of reaction r13 were derived : 85(-19)(+9)% C3H2 plus H, and 15(-9)(-19)% C3H plus H-2. Concomitantly with the identification of reaction r25a as dominant C2H source, the rate constant of the reaction C2H + O (r19) was determined relative to the well-known kinetic coefficients of C2H + C2H2 and C2H + O-2 : k(19) = (9 +/- 4) x 10(-11) cm(3) molecule(-1) s(-1) at 600 K. It is suggested that a sizeable fraction of the ethynyl radicals formed in fuel-rich hydrocarbon flames is produced likewise by oxidation of C3Hx radicals (x = 1-3) that arise in the fast reactions of CH(X(2) Pi) and CH2(a(1)A(1)) with C2H2.