In orthorhombic Ba6Mg7F26, the Ba2+ ion can be partially replaced by Sr2+ to form a mixed, disordered compound. This new compound has a refined composition of Ba5.24 (4)Sr0.76 (4)Mg7F26. The volume decreases after the partial substitution from 1074.64 (10) to 1064.31 (11) Å3. The structure has two barium sites, Ba1 on Wyckoff site 8(l) with Cs symmetry and a coordination number of 12 + 1, and Ba2 on site 4(j) with C2v symmetry and a coordination number of 12; only the Ba2+ on site 4(j) is partially replaced by Sr2+. The distorted octahedral fluoride environment around magnesium shows a tendency to become more irregular in the Sr-substituted compound.