Energy levels of OH- in KCl crystals are evaluated from transitions due to resonant absorption of electromagnetic energy. These transitions are interpreted in a consistent way through a model in which the OH- moves in a 3-dimensional set of harmonic potentials. With respect to the lowest set of energy levels, i.e., those due to tunneling, microwave absorption measurements show that the splitting factor equals 5.5±1.1 kMc/sec and the apparent dipole moment of this system equals 3.3±0.6 Debye units 0.69±0.12. Furthermore, a combination of the present microwave absorption results with infrared spectra shows that the center of mass of the OH- ion is displaced 0.3 from the halide lattice site. Before carrying out the above calculations, the effects of actual experimental conditions upon microwave-power absorption were evaluated. Such evaluation, including theoretical verification, gives the following results: Saturation of the microwave transitions can be readily achieved in a re-entrant cavity. From this effect, the relaxation time is shown to be > 10-7 sec at 1.4°K and is limited by single-phonon interactions in the 1.4 to 6°range. Phonon interactions produce negligible broadening below 6°K. OH- concentrations 20 ppm lead to line broadening via a simple (electrostatic) dipole-dipole interaction. In samples with ≤1.4 ppm of OH- ions, the linebreadth is due to the (∼107 dyne/cm2) stress of grown-in dislocations. Having established the above facts, their deleterious effects were minimized before fitting the data with our model. © 1967 The American Physical Society.