Using small angle neutron and X-ray scattering measurements, the temperature dependence of the medium range structure Of amorphous Si24.4C42.4N33.2 ceramics was investigated. Contrast variation by the isotopic substitution method (N-nat and N-15) showed that the observed SAS signal is caused by two separate phases of amorphous Si3N4 and amorphous carbon within the material. The measured invariant remains constant over the whole observed temperature range, whereas the correlation volume increases with increasing temperature.