Using modulated beam line-of-sight threshold ionization mass spectrometry (LOS-TIMS) we measured absolute O, O-2, and Ar densities, and the average neutral-gas temperature in an O-2/Ar electrical discharge as a function of pressure in the plasma chamber and the mole fraction of Ar in O-2: the pressure and mole fraction range was 25-200 mTorr and 0-0.90, respectively. Although LOS-TIMS is a versatile tool for measuring absolute radical densities, it requires careful vacuum design and calibration to account for various sources of error such as the contribution to the quadrupole mass spectrometer (QMS) ion current from the background gases, the ion mass-to-charge ratio dependent sensitivity of the various QMS components, and space-charge limitations in the QMS ionizer. In addition, collisions within the molecular beam extracted from the discharge must be taken into account particularly for higher plasma chamber pressures (> 75 mTorr). In our measurements, these effects are carefully considered and the consequences of ignoring them are discussed. The O atom density increases with pressure and O-2 mole fraction in the feed gas and is in the range of 2.1 x 10(18)2.6 x 10(19) m(-3). At low pressures, our measurements show that the O-2 translational temperature is higher than that for Ar. (C) 2004 American Vacuum Society.