The molecular structure of SbI3 and BiI3 have been studied by gas-phase electron diffraction and infrared Spectroscopy, BiI3 has been studied by electron diffraction, and SbI3 has been studied by infrared spectroscopy for the first time. Both molecules are pyramidal (C-3v symmetry), and their geometries are characterized by the following bond lengths (r(g)) and bond angles (angle(alpha)) : Sb-I 2.721 +/- 0.005 Angstrom, I-Sb-I 99.0 +/- 0.30, Bi-I 2.807 +/- 0.006 Angstrom, and I-Bi-I 99.5 +/- 0.3 degrees. Three of the four normal modes of vibration were observed for both molecules : SbI3, v(2) = 74.4 cm(-1), v(3) = 193.6 cm(-1), and v(4) = 54.6 cm(-1); BiI3, v(2) = 59.7 cm(-1), v(3) = 163.5 cm(-1), and v(4) = 47.0 cm(-1). A joint electron diffraction/vibrational spectroscopic analysis was also carried out, from which the harmonic equilibrium geometries were estimated. Force field parameters from a normal coordinate analysis are reported for both molecules.