Amorphous 11B31,9C36,8N31,3 ceramic samples were produced by thermolysis of poly(borosesquicarbodiimide) of idealized formula [B2(NCN)3]n at 1100°C in inert gas atmosphere. Small angle scattering (SAS) experiments reveal, that the as-obtained ceramics are chemically inhomogeneous. Using small angle neutron and X-ray scattering measurements, the temperature dependency of the medium range order was investigated. The SAS curves show a strong temperature dependence of the Guinier-radius (R
) of the scattering regions. Contrast variation by X-ray and neutron diffraction shows that the observed SAS signal is caused by amorphous BC2N enclosing amorphous boron-nitride segregations. The fact, that the measured invariant remains constant over the whole temperature range supports the view of complete phase separation.