The variation of the molar volume, V, and the glass transition temperature, TQ with the average coordination number, m, for twenty-three glass compositions. belonging to the Ge15SbxS85-x and Ge20SbxS80-x families of the Ge-Sb-S system, is reported and discussed. The V-m dependence for the Ge15SbxS85-x family shows a minimum in V at m=2.4 which is attributed to the floppy-to-rigid transition in network glasses. For both families of the glasses examined, the V-m and Tg-m dependences show peaks at m=2.52 for Ge15SbxS85-x and at m=2.56 for Ge20SbxS80-x, corresponding to the respective stoichiometric composition for each family. These features are found to be consistent with topological and chemically ordered covalent network models proposed for the structure of these glasses.