This paper describes a newly developed long-path differential-optical-absorption-spectroscopy instrument used for the measurement of tropospheric OH radicals. The instrument consists of a high resolution echelle spectrometer in conjunction with a multiple-reflection cell of 38.5 m base length and a UV laser light source that provides a spectral line width of 0.41 nm. Local in situ absorption measurements at total path lengths of either 1.85 or 3.1 km can be performed. The simultaneous observation of six atmospheric OH rotational absorption lines (Q1(2), Q21(2), R2(2), Q1(3), Q21(3), and P1(1)) around 308 nm allows OH measurements with high specificity. A new method to accurately determine the precision and the detection limit of each individual OH measurement data point is presented. Presently, a 2&sgr;-detection limit of 1.5¿106 OH cm-3 is achieved (based on 1.85 km absorption path length and about 6 min integration time), which corresponds to a minimum detectable optical density of 2.5¿10-5. The absolute instrumental accuracy was calculated to be better than 6.5%, which emphasizes the qualification of the long-path absorption technique as an absolute method. Examples of field experiments are reported to illustrate the present performance. ¿ 1997 American Geophysical Union