The compositions Sc 5 M 3 - x M' x (M=Al or Ga; M'=Sn, Sb or Te) were prepared by high-temperature solid-state techniques, and their Mn 5 Si 3 -type structures were either identified by powder X-ray means or determined by single crystal X-ray diffraction (hexagonal P6 3 /mcm (No. 193), Z=2). Each Al or Ga (M) atom type exhibits mixed occupancy with Sn, Sb, or Te (M') over different composition regions. For systems annealed at 1100-1575 o C, single crystal data indicate that the phase widths of Sc 5 M 3 - x M' x extend over the ranges x=1.38(6)-2.25(2), 0.83(1)-0.96(1), 0-2.25(6), and 0-1.25(3) for Al/Sb, Al/Te, Ga/Sb, and Ga/Te, respectively. Powder X-ray data on the Sn systems shows phase width ranges of x=~1.2-3.0 with Al and 0.0-3.0 with Ga, respectively. No interstitials in the Mn 5 Si 3 -type host were seen. The series of mixed compounds illustrate regular effects of substitution of larger and electron-richer M' atoms in the flexible structure. The lattice constant trends follow Vegard's law, with natural increases of a (b) and V with increasing x, but with small and irregular changes in c. Physical property measurements show that many of the compounds display metallic characteristics, with positive temperature-dependent resistivity and Pauli-like paramagnetism. The structure of Sc 5 Sb 3 is reassigned to the Y 5 Bi 3 -type (Pnma).