The dynamic flow properties and compaction characteristics of mannitol and mannitol-sodium carbonate mixtures were measured using a Freeman FT4 powder rheometer. The results showed that the mixtures containing up to 30% sodium carbonate had better flow properties and improved compaction characteristics when compared with mannitol alone. The mixtures were also analyzed using a shear cell, which gave flow functions that varied little between the samples. The feasibility of combining a more expensive pharmaceutical excipient with a cheaper ingredient is demonstrated, without compromising desired powder characteristics. The study may also provide a useful method for assessing the suitability of new formulations for application as direct compression bases, as part of a wider range of powder flow tests.