The application of Cavity Ring Down Spectroscopy for the quantitative determination of CH densities in atmospheric flames has been investigated. Two different atmospheric flames have been studied: a premixed burner stabilised flat CH4/air flame and an oxyacetylene flame from a welding torch, which is used for diamond deposition. For the methane flame density profiles of CH are obtained and compared with densities calculated by De Goey and Van Oijen (1998). The temperature is derived from the measured Boltzmann distribution. The data from the oxyacetylene flame are compared to LIF profiles measured by Klein-Douwel et al. (1995).