Transition metal oxynitrides with perovskite type structures are an emerging class of materials with optical, photocatalytic, dielectric and magnetoresistive properties that may be sensitive to oxide/nitride order, but the anion ordering principles have been unclear. We recently studied the representative compounds SrMO2N (M = Nb, Ta) using neutron (D2B@ILL) and electron diffraction data. This revealed a robust partial anion order in high temperature pseudo-cubic phases, which directs the rotations of MO4N2 octahedra in room temperature superstructures. We propose here to study SrTaO2N and two related Ta oxynitride perovskites to higher temperatures, ~1200 C to establish whether a transition to a truly cubic phase occurs and to confirm and extend the recently discovered principles.