Sideband optical-optical double resonance Zeeman spectroscopy is applied here to studies of the transition metal hydrides NiH, PdD, and PtH. For both NiH and PtH, in addition to obtaining Zeeman information on the ground and several excited electronic states, extremely small ground state proton hyperfine splittings have been resolved. For PdD, sub-Doppler spectra of the X Omega = 0.5 ground state have been recorded even though the upper state in the optical transition is strongly predissociated, For all three radicals, the double-resonance Zeeman studies reveal that the g values in each state deviate from their nominal Hund's coupling case (a) or (b) value with increasing rotational quantum number J. This Zeeman information, in addition to providing a sensitive confirmation of electronic assignments, also provides direct evidence of electronic state mixing (which is not present in ordinary non-Zeeman spectra) and useful in modeling the interactions between electronic states. (C) 1997 American Institute of Physics.