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At temperature T>-2200 K, ketene (CH2CO) is known as a convenient, reproducible and well-defined CH2source. In experiments performed behind reflected shock waves, the subsequent reactions of CH2were monitored by concentration measurements of various product species, including C, H, and CH, using atomic resonance absorption and ring dye laser absorption spectroscopy. The time histories of these species were investigated in various initial ketene (5–75ppm)/argon mixtures in the temperature range 2200–3400 K at pressures 0.7≤p≤2.4 bar. From the concentration slopes at t≈0, apparent rate coefficients for C, H, and CH formation were determined, but only the C concentration profiles at early reaction times have direct kinetic significance. From these data, the rate coefficient of the thermal decomposition reactionCH2+M⇋k2C+H2+Mk2=1.60×1014exp⁡(−32,200K/T)cm3mol−1s−1(R2)