A weak field DNP study of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (4-hydroxy-TEMPO), 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxy (4-amino-TEMPO), and 2,2-Di(4-tert-octylphenyl)-1-picrylhydrazyl (DPPH) free radicals in various solvents was performed at 1.53 mT using a double resonance spectrometer. pi transitions, in which the RF field has a component perpendicular to the static magnetic field B-0, and sigma transitions, in which the RF field has a component parallel to B-0, were recorded. In order to observe the a transitions a new probe head was designed. The sigma transitions were obtained at the frequencies expected from theoretical calculations. However, the results obtained for the observed enhancements deviated from the values expected theoretically. (C) 1998 Academic Press.