Transition metal oxynitrides with the perovskite structure type are an emerging class of functional materials with a variety of interesting optical, photocatalytic, dielectric and magnetoresistive properties that may depend upon anion order. However, the oxide/nitride ordering principles in such materials were unclear until a recent study of the representative compounds SrMO2N (M = Nb, Ta) by neutron and electron diffraction revealed a robust partial anion order. A PDF study is requested here to explore this local order. Three representative Ta oxynitrides with different structure and tilt systems will be studied.