Magnetization of SrLaMnO4+x (0 ≤ x ≤ 0.2) and neutron diffraction of stoichiometric SrLaMnO4 reveal that oxygen excess SrLaMnO4+x is ferromagnetic and stoichiometric SrLaMnO4 is an antiferromagnet with a wave vector [1/2, 1/2, 0] and TN = 180 K. In SrLaMnO4+x, ferromagnetic clusters are associated with coupling between Mn3+ and Mn4+ ions produced in antiferromagnetic SrLaMnO4