Cavity ring-down laser spectroscopy (CRDS) is used to measure the OH concentration profile and the rotational temperature profile in low-pressure (30 torr) methane/ air flames. Very high sensitivity (2 × 1010 molecules/cm3) and large dynamic range (more than 20000) are demonstrated. CRDS also provides accurate temperature measurements, with statistical errors less than 2%. Measured concentration profiles are in reasonable agreement with calculated values. It is observed that in the preflame zone (where the temperature is about 1000 K), the OH concentration at the first vibrational excited state is about seven times larger than the equilibrium OH(υ″ = 1) concentration at this temperature.