The measurement of radical densities in atmospheric-pressure plasmas has gained increasing importance in recent years in view of their crucial role in many applications. In this paper we present absolute OH density measurements by broadband UV absorption in diffuse atmospheric-pressure RF glow discharges in mixtures of He and H 2O. The use of a 310nm light-emitting diode as a light source and a very high resolution spectrometer (2.6pm resolution) made the estimation of the total OH density possible by simultaneously measuring the absorption rates of different spectrally resolved rotational lines of the OH(AX) transition. For different RF powers and water concentrations, OH densities and gas temperatures ranging between 6×10 19and 4×10 20m 3 and 345 and 410K, respectively, were obtained. The gas temperature T g was also measured by three different methods. T g deduced from the rotational temperature of N 2(CB) emission, nitrogen being present as a trace impurity, provided the most reliable value. The rotational temperature T r of the ground state OH(X) presented values with a maximum deviation of 25K compared with T g. To obtain the gas temperature from the emission intensities of OH(AX) rotational lines, the recorded intensities of different lines must be corrected for the effect of self-absorption inside the plasma.