Spectra due to the A2Σ+–X2Π transition of the hydroxyl radical in electric fields of over 300 000 V per cm have been obtained. The dipole moment of the A2Σ+ ν = 0 state of OH has been determined to be (1.98 ± 0.08) D. From the variation of the dipole moment with vibrational quantum number in the 2Π state, the transition probability for the pure vibrational transition ν = 1 →ν = 0 has been estimated to be A10 = 80s−1.