Abstract: RF modulation spectroscopy of a near-infrared tunable laser diode source is used to determine the oxygen concentration in a sample medium. A reference cell containing a known concentration of oxygen is used to calibrate the apparatus as well as to lock the laser on an oxygen absorption line. The temperature of the reference cell is monitored from which the pressure in the reference cell can be determined. Both the temperature and the pressure in the sample cell are monitored, either directly using independent transducers or indirectly using spectroscopic techniques. The oxygen content of the sample is determined, correcting for both temperature and pressure effects.