The cubic intermetallic phase Y4Mn1-xGa12-yGey (x = 0-0 26, y = 0-4 0) has been isolated from a molten gallium flux reaction It presents a rare example of a system where ferromagnetism can be induced by controlling the vacancies of the magnetic centers. The Y4PdGa12 type crystal structure is made up of a corner-sharing octahedral network of Ga and Ge atoms with Mn atoms at the centers of half the octahedra and Y atoms in the voids At the highest Ge concentration, y = 4.0, the Mn site is nearly fully occupied, x = 0.05, and the samples are paramagnetic At a lower Ge concentration, y = 1 0, Mn deficiency develops with x = 0 10 Surprisingly, strong ferromagnetism is observed with T-c = 223 K. When Ge is excluded, y = 0, Mn is substantially deficient at x = 0.26 and ferromagnetism is maintained with a T-c of similar to 160 K. In addition, a 6-fold modulated superstructure appears owing to an ordered slab-like segregation of Mn atoms and vacancies. Corresponding bond distortions propagate throughout the octahedral Ga network Structure-property relationships are examined with X-ray and neutron diffraction, magnetic susceptibility, and electrical resistivity measurements