The effect on AIN ceramic of the addition of V2O3, Vb(2)O(3), Er2O3 and CaO were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal conductivity measurements. The effect of grain boundary segregation and second phase distribution on the thermal conductivity are discussed. The Er2O3-CaO- and the Yb2O3-CaO- AIN ceramics have a higher thermal conductivity than the CaO- and the Y2O3-CaO- AIN ceramics. This is explained on the basis of the free energy of formation (Delta G degrees), the vaporization of the sintering additives and the microstructural development. Oxidation of freshly cleaned surfaces of those AIN ceramics was studied.