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The hyperfine structure of OH in the ^4 2 2 1/^+ state has been measured by exciting a molecular beam with cw intracavity frequency-doubled dye-laser radiation. The splittings are 778 MHz for N' = 0 and 200 to 500 MHz for 1 *£ N' *s 5. The values for the hyperfine andp doubling are &+|c = 777.8±2.0 MHz, c -165.8±2.8 MHz, and y = 7.13± 0.03 GHz. Also the hyperfine splitting of OD in the N' =0 state has been measured yielding b + Jc = 119 ±2 MHz.The hydroxyl radical (OH) is one of the fundamental molecules in chemistry, physics, and astrophysics, It is the simplest diatomic openshell system, readily accessible for a wide range of spectroscopic techniques (optical, 1 infrared, 2 microwave, 3 and molecular beam-maser and -resonance 4,5 ) and accurate calculations. The radical plays an important role in laboratory and atmospheric chemistry and its maser emission in interstellar clouds forms a major unsolved problem in astrophysics,In the past considerable effort has been spent to unravel the structure and properties of OH a The structure of the ground electronic state (X 2 n), its vibrational and rotational levels, the p doubling, and the hyperfine structure are all well known. 6 This is, however, not the case for the first excited electronic state A 2 S 1/2 + where only the rotational and A-doubling energies are known from old optical work. The hyperfine structure due to the interaction between the magnetic moment 7=| of the hydrogen nucleus and the electronic spin has not been measured yet 0 In level-crossing experiments German et al 0 7 * 8 determined the hyperfine structure of OD in the A 2 S 1/2 + state from which they calculated the splittings for OH. The estimated values are on the order of 300 MHz which is beyond the resolution of conventional Doppler-limited spectroscopy (4 GHz at 300 K).The knowledge of the hyperfine structure is important not only for a theoretical understanding of the excited structure but also for the uv excitation models of interstellar OH radicals. As proposed by Litvak et al., 9 uv pumping might cause a population inversion between the A-doublet levels in the rotational ground state 2 n 3/2 , J=|, leading to the observed maser emissions. Turner 10 has

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