A new family of nonstoichiometric materials, GaV4-xMO xS8 (0=x=4), has been prepared and characterized by high-resolution neutron powder diffraction, laboratory and synchrotron X-ray powder diffraction, thermogravimetry, and SQUID magnetometery. All materials adopt a cubic (F43m) structure at ambient temperature. The end-member phases GaV4S8 and GaMo4S8 exhibit a distortion to rhombohedral (R3m) symmetry at low temperatures. High-resolution powder neutron diffraction reveals that the phase transition is incomplete and that both cubic and rhombohedral phases coexist below ca. 40 K. Magnetic susceptibility data indicate that at low temperatures, the end-member phases undergo a transition to a ferromagnetic state in which the metal-metal exchange interactions within cubic B4S4 clusters are predominantly antiferromagnetic in origin. At intermediate compositions (1=x=3) a cubic structure persists to 4 K, and materials remain paramagnetic to this temperature. The suppression of the rhombohedral distortion may be responsible for the absence of magnetic ordering observed in the nonstoichiometric members of the GaV4-xMoS8 series. © 2007 American Chemical Society.