Sr3Au8Sn3 was synthesized through fusion of a stoichiomettic amount of pure metals at 800 degrees C and annealing treatments at lower temperatures. Single-crystal X-ray diffraction analyses revealed that Sr3Au8Sn3 has a La3Al11-type Immm structure (a = 4.6767(8) angstrom, b = 9.646(2) angstrom, c = 14.170(2) angstrom, Z = 2) if annealed at 550 degrees C and above but a Ca3Au8Ge3-type structure (Pnnm, a = 9.6082(8) angstrom, b = 14.171(1) angstrom, c = 4.6719(4) angstrom, Z = 2) if annealed at 400 degrees C. The transition occurs at about 454 degrees C according to DTA data. Both structures feature columns of Sr-centered pentagonal and hexagonal prisms of Au and Sn stacked along the respective longest axial directions, but different "colorings" of the polyhedra are evident In the high temperature phase (Immm) all sites shared between the two prisms adopt 50:50 mixtures of Au/Sn atoms, whereas in the low temperature phase (Pnnm) Au or Sn are completely ordered. A Klassengleiche group-subgroup relationship was established between these two structures. LMTO-ASA calculations reveal that Delta E for the disorder-to-order transformation on cooling is driven mainly by optimization of the Au-Au and Au-Sn bond populations around the former mixed Au/Sn sites, particularly those with extremely short bonds at the higher temperature. These gains also overcome the smaller effect of ordering on the entropy decrease.