OH multipass absorption is an absolute and spatially resolving method for the in situ monitoring of tropospheric OH. The basis of the method is differential optical absorption spectroscopy. The absorption path in ambient air is folded within a 6 m White cell type multiple reflection system with an open path setup. SO2, CH2O, and naphthalene are measured at 308 nm simultaneously with OH by means of the first harmonic of the laser radiation. NO2 can be measured using the fundamental at 616 nm, respectively. The detection limit of the reported instrument is 2--5¿105 OH cm-3 corresponding to a signal to noise ratio of better than one or two for a 1-min monitoring time dependent on the boundary conditions. The performance of the method is demonstrated by tropospheric OH measurements achieved during a field campaign at the Taunus Observatory on the Kleiner Feldberg in the vicinity of Frankfurt/Main.