Monochlorogallane, synthesized by the metathesis of gallium(III) chloride with an excess of trimethylsilane at ca. 250 K, has been characterized by chemical analysis, by its IR, Raman, and H-1 NMR spectra, and by the products of its reaction with trimethylamine. The vibrational spectra of the vapor species isolated in solid Ar, N-2, or CH4 matrixes at ca. 12 K imply the presence of only one species, viz. the dimer with an equilibrium structure conforming to D-2h symmetry. The structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements augmented by the results of ab initio molecular orbital calculations. An equilibrium structure with D-2h symmetry has been assumed in the analysis of the electron diffraction pattern. However, as the molecule has a very low frequency Ga(mu-Cl)(2)Ga ring-puckering mode, a dynamic model was used to describe it with the aid of a set of pseudoconformers spaced at even intervals (Delta delta = 5 degrees, delta(max) = 20 degrees) around the ring-puckering angle delta and Boltzmann-weighted according to a quartic potential V(delta) = V(4)delta(4) + V(2)delta(2). The differences in bond distances and angles between the different pseudoconformers were constrained to the values derived from the ab initio calculations employing second-order Moller-Plesset (MP2) methods (with all the electrons included in the correlation calculations) and a 6-311G(d) basis set. The results for the weighted average of the principal distances (r(alpha)) and angles (angle(alpha)) (with estimated 1 sigma uncertainties) from the combined GED/ab initio study are r(Ga-Cl) 2.350(2) Angstrom, r(Ga-H) 1.523(20) Angstrom, angle Cl-Ga-Cl 89.7(5)degrees, angle H-Ga-H 135.1 degrees, V-4 3.0 x 10(-6) kcal mol(-1) deg(-4), and V-2 6.0 x 10(-4) kcal mol(-1) deg(-2).