The freezing point depression of cryolite (Na3AlF6) by the addition of Al2S3 and FeS was investigated. It was found that for contents of up to 10 wt.% Al2S3, it brings into the melt three new species. X-ray analysis of solidified melts of the system Na3AlF6-Al2S3 showed that it contained chiolite, Na5Al3F14 and Na2S. Chiolite originates from a reaction between Na3AlF6 and AlF3. This suggests that the system Na3AlF6-Al2S3 is a part of the reciprocal system NaF, AlF3//Na2S, Al2S3. The solubility of FeS in cryolite melt is so low that it cannot be determined by the thermal analysis. When FeO is added to the Na3AlF6-Al2S3 melt, Fe2+ cations and S2- anions react under the formation of solid FeS. A similar reaction was observed for Ni2+ and S2- ions. (C) 2003 Elsevier B.V. All rights reserved.