Using the technique of Fourier transform microwave spectroscopy in the range 1-14 GHz Q-branch rotational transitions have been observed for (SbH3)-Sb-121 and (SbH3)-Sb-123 in the ground, upsilon (2) = 1 and upsilon (4) = 1 vibrational states with an accuracy of 0.1-100 kHz. A(1)-A(2) splitting transitions for k=+/-3 in the ground state, k = +/-3, +/-9 in upsilon (2) = 1, and for kl = +1, -2, +4 in the upsilon (4) = 1 vibrational state have been observed. We also measured perturbation-allowed transitions with selection rule Deltak = +/-3 in the ground vibrational state for k = +/-1 <----> -/+2, with selection rules Delta (k-l) = 3,6,9 in the upsilon (2) = 1 state for k = +/-1 <----> -/+2, 0 <----> +/-3, +/-2 <----> -/+4, +/-2 <----> -/+7, and 0 <----> +/-9 and in the upsilon (4) = 1 state for kl = +2 <----> +3, +2 <----> +6, and +3 <----> +5. The transitions show hyperfine structures due to the quadrupole and spin-rotation coupling of the nuclear spin I-Sb and the rotational angular momentum J. Hyperfine structures in the dyad upsilon (2) = 1, upsilon (4) = 1 have been analyzed using an effective Hamiltonian extended to higher order spin-rotation coupling terms and including spin-vibration coupling. A total of 21 hyperfine parameters has been determined for each isotopomer including quadrupole and spin-rotation constants of the (Deltal,Deltak) = (0,3), (2,2), and (2,-1) interactions. A similar analysis has been performed for the ground vibrational state yielding 7 (6) hyperfine parameters for (SbH3)-Sb-121 ((SbH3)-Sb-123) including the (0,3) interaction constants. Splittings of transitions between E-states involving basis states with k = +/-1 have been observed in the ground, upsilon (2) = 1 and upsilon (4) = 1 vibrational states. This splitting has been unequivocally explained as lifting of parity degeneracy by proton hyperfine interactions. From the analysis of the ground state hyperfine doublets, tensorial constants of the H spin-rotation coupling and the Sb-H spin-spin interaction have been accurately determined. (C) 2001 American Institute of Physics.