A long path laser absorption technique for measuring tropospheric OH is presented. The exptl. set-up includes an excimer laser which serves as a pump source for a frequency-doubled dye laser, a collimator and a delayer to permit the joining of the laser beams to produce a 'double-beam', a 10 * beam expander and a 15 * Cassegrainian telescope, a 50-cm plane mirror, a Gregorian telescope, ref. cell for OH-fluorescence excitation of a fraction of the frequency-doubled light, and a photomultiplier. The method makes use of 2 short laser pulses of 0.0006 nm bandwidth (anal.) and 0.1 nm band width (ref.), resp. Signal intensities showing random fluctuation, averaging over 2500 signals, leads to an accuracy of 10-4 corresponding to an OH-concn. of 2 * 10-5 /cm3. Since beam displacements cause data losses ? estd. 80% and absorption measurements will be made on and off the OH-line, a single measurement period takes .apprx.20 min.