Abstract The collisional quenching of OH A 2Σ( v′ = 1) by H 2O was determined from laser-induced fluorescence measurements in the burned-gas region of stoichiometric H 2/O 2/Ar flames at low pressure (19 and 38 Torr). The collisional quenching was measured as a function of the initial laser-populated rotational level for N′ =4, 5, 8, 9, 10 and 11). The quenching cross section after laser excitation to N′ = 4 is 32±2.5 Å 2 at a temperature of 1048 K, approximately 40% of its value at room temperature. The quenching cross section decreases by 17% as the initial laser-excited rotational level is increases from N′ = 4 to N′ = 11. This variation in quenching rate has a significant effect on the determination of OH rotational temperature from laser-excitation spectra. Accounting for the variation of quenching with rotational level reduces the rotational temperature by 10% at 1100 K. The collisional quenching cross section of OH A 2Σ( v′ = 1) by H atoms was calculated to be 10±3 Å 2 for N′ = 5 at a temperature of 1048 K.