We report on the magnetic properties of single-crystalline SrLaFe O4 and Sr1.1 La0.9 Fe O4. Both compounds crystallize in the tetragonal K2 Ni F4 structure (I4mmm) with the La and Sr atoms randomly distributed over the 4e crystallographic sites of this structure. SrLaFe O4 orders antiferromagnetically below 350 K, and Sr1.1 La0.9 Fe O4, orders antiferromagnetically at about 150 K. Both compounds exhibit magnetic structures described by orthorhombic Cmca -type symmetry and two magnetic propagation vectors q1 = (1 2, 1 2, 0) and q2 = (- 1 2, 1 2, 0), representing the two possible magnetic domains. A collinear antiferromagnetic arrangement of the Fe3+ moments confined to the tetragonal basal plane is found. Bulk magnetic studies on SrLaFe O4 show that the properties of this compound are highly anisotropic, and there is evidence for additional magnetic transitions at lower temperatures (30 and 90 K). Low-temperature neutron diffraction data were taken to gain some insight into the nature of these transitions.