The rate constant for the CH+H2O reaction has been measured in a fast-flow reactor to be (0.9±0.4)×10−11cm3molecule−1s−1 at 330K. The product branching ratio of H atoms probed by resonance fluorescence is determined to be 100%. Quantum chemistry and statistical calculations show that the rate limiting process is the isomerisation of the HCOH2 adduct to H2COH and allow us to propose k=(2.8±1.4)×10−11(T/300)−1.22±0.07exp(−(12±12)/T)cm3molecule−1s−1 in the range 100–700K for the rate constant of CH+H2O→H+H2CO and H2+H+CO.