Nitridosilicates and oxonitridosilicates (sions) gain more and more importance as host lattices for rare-earth doped phosphors in light emitting diodes. By means of neutron-diffraction we want to determine structural details of three sions, whose luminescence properties are particularly interesting. X-ray methods do not allow the complete determination of these structures, as the scattering length of N is very small as compared to the heavy elements present and furthermore does not differ from that of O. As possible N/O ordering results in a different environment of the rare-earth metals used for doping, it has a significant impact on the luminescence properties. The knowledge of the O/N distribution would give us an opportunity to control these properties. (Oxo)nitridosilicate, luminescence, light emitting phosphors, O/N distribution, mixed site occupancy