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 Ba_{5.24 (4)}Sr_{0.76 (4)}Mg₇F₂₆. The volume decreases after the partial substitution from 1074.64 (10) to 1064.31 (11) ų. The structure has two barium sites, Ba1 on Wyckoff site 8l with C_s symmetry and a coordination number of 12+1, and Ba2 on site 4j with C_2v symmetry and a coordination number of 12; only the Ba²⁺ on site 4j is partially replaced by Sr²⁺. The distorted octahedral fluoride environment around magnesium shows a tendency to become more irregular in the Sr-substituted compound.