The separation of polar compounds by supercritical-fluid chromatography is difficult with carbon dioxide as mobile phase. In this work, we tested mixtures of ammonia with sulphur hexafluoride as mobile phase and we have compared the solvating power of these mixtures with carbon dioxide for five solutes. The results show that the solvating power of sulphur hexafluoride-ammonia is weak in comparison with carbon dioxide, except for aniline. Ammonia plays only a role as silanol masking agent. Furthermore, ammonia is harmful for the pumping system.