The present proposal on SrFeO3-x is motivated by our recent inelastic neutron scattering measurements, where three incommensurate magnetic phases with propagation vectors k2 = (0.20, 0.20, 0.20), k5 = (0.30, 0.30, 0.75), and k6 = (0.79, 0.79, 0) have been identified in the compounds with (3-x) > 2.75. However, in our neutron powder diffraction studies these magnetic phases could not be identified, probably due to a strong overlap of reflections of coexisting magnetic phases. Furthermore, single-crystal diffraction experiments could not clearly confirm the existence of magnetic reflections. In the proposed experiment, we plan to measure the reciprocal lattice maps using the E2 Flat-cone diffractometer for the compounds with (3-x) = 2.80 and 2.84 . In addition, for the investigation of the crystal structures of the compounds with (3-x) = 2.80, 2.84, and 2.86, we propose to collect the full powder data sets using the E9 diffractometer.