Cross-section transmission electron microscopy was used to investigate the microstructure of polycrystalline silicon nitride (Si3N4) and aluminum nitride (AIN) following 2 MeV Si ion irradiation at 80 and 400 K up to a fluence of 4×1020 ions/m2(maximum damage of ∼ 10 displacements per atom, dpa). A buried amorphous band was observed at both temperatures in Si3N4 in the region corresponding to the peaks in the implanted ion and displacement damage. From a comparison of Si3N4specimens irradiated at different fluences, it is concluded that the amorphization is primarily controlled by the implanted Si concentration rather than the displacement damage level. Si3N4 amorphization did not occur in regions well-separated from the implanted ions for doses up to at least 3 dpa at 80 K, whereas amorphization occurred in the ion implanted region (calculated Si concentration >0.01 at.%) for damage levels as low as ∼0.6 dpa. The volumetric swelling associated with the amorphization of Si3N4, is <10%. Amorphization was not observed in any of the irradiated AIN specimens. A moderate density of small (∼3 nm) defect clusters were observed in the crystalline damaged regions of both the Si3N4 and AIN specimens at both irradiation temperatures. Aligned network dislocations were also observed in the AIN specimen irradiated to high dose at 80 K.