Phases in the LaxSr2-xCoGaO5+δ (0.5 ≤ x ≤ 1) series adopt anion-vacancy ordered brownmillerite-type structures. The x = 0.5 and x = 0.6 phases show only small deviations from the ideal A2B2O5 brownmillerite stoichiometry and exhibit paramagnetic behaviour above 100 K, with magnetic susceptibilities indicative of a high-spin, S = 2 configuration for the Co(3+) centres present. In contrast, lanthanum-rich phases (x ≥ 0.7) incorporate increasing amounts of 'additional' oxygen and show much smaller paramagnetic susceptibilities, consistent with the Co(3+) centres present adopting low-spin S = 0 spin states. The evolution with lanthanum concentration of both the oxygen stoichiometry and the magnetic susceptibility of phases is rationalised on the basis of the lattice strain which builds across the compositional series. This lattice strain acts to couple the Co(3+) spin state to the oxygen stoichiometry in the range 0.65 < x < 0.85 giving materials oxygen sensing capability.