The vapor phase species responsible for the transport of impurities in the sublimation-recondensation growth of bulk AlN crystals was predicted by thermodynamic analysis. AlN powder containing oxygen was investigated in Al-O-N system for an inert reactor. Dialuminum monoxide (Al2O) is strongly favored over all other possible oxygen containing species including NO and NO2. For AlN crystal growth in a graphite furnace, the Al-O-C-N system was studied. CO is the main species containing carbon and oxygen, and has a partial pressure more than one hundred times higher than all other carbon or oxygen containing species. Its partial pressure even exceeds that of Al vapor. Pure AlN growth on SiC seed was represented in the Al-N-Si-C system. SiC is not stable at high temperatures, the presence of nitrogen accelerates the decomposition of the SiC, and the most probable volatile silicon and carbon species originating from the SiC seed are Si, CN and C2N2.