The novel oxide Sr 2 Co 2 - x Ga x O 5 with brownmillerite-type structure has been synthesized in the compositional range 0.3=<x=<0.8. Rietveld refinements using neutron powder diffraction data have been performed for the end compositions, x=0.3 and 0.8. The structure is best described in the space-group Icmm (no. 74) with unit cell parameters a=5.5678(6), 5.6126(7)A, b=15.749(2), 15.733(2)A and c=5.4599(6), 5.4559(7)A for the x=0.3 and 0.8 compositions, respectively. The compounds were found to be G-type antiferromagnetic with the magnetic moments parallel to the c-axis. High-temperature magnetic susceptibility measurements confirmed the samples to be antiferromagnetic with Neel temperatures T N =505, 468 and 423K for the x=0.3, 0.5 and 0.8 samples, respectively. High-resolution transmission electron microscopy and electron diffraction studies confirmed the I-centred structure and revealed the presence of disorder.