Polycrystalline samples of bilayered brownmillerite-like Ca 2.5 Sr 0.5 GaCo 0.15 Mn 1.85 O 8 and Ca 2.5 Sr 0.5 Ga 1.2 Mn 1.8 O 8 have been prepared and characterised by magnetometry and neutron diffraction over a wide temperature range. The structural chemistry and magnetic properties are compared to those of Ca 2.5 Sr 0.5 GaMn 2 O 8 . Ga enrichment has a significant effect on the former but not on the latter, whereas changes in both occur when paramagnetic Co 3+ cations enter the parent phase on the 4-coordinate sites. The coupling between the environment around the 4-coordinate cations and the transition to an antiferromagnetic ordered state that was observed in Ca 2.5 Sr 0.5 GaMn 2 O 8 is not apparent in the cation-substituted compositions, although both show long-range antiferromagnetic order at low temperatures.