Powder filled hard gelatine capsules are an established dosage form. There are relatively few literature reports studying the filling behaviour of tamp filling machines due to the large variety of dosing systems available for filling capsules. The aim of the study was to investigate the effect of excipient type and levels and to identify relationships between powder flow, filling performance, disintegration and dissolution of powder mixtures using the tamp filling principle. Two studies using two model drugs were carried out. The first study used theophylline as the model drug due to its poor solubility. The second study used ibuprofen due to its low melting point. The fillers used were lactose and microfine cellulose respectively. Partially pregelatinised starch and magnesium stearate were used in both studies. A central composite factorial design was employed in order to identify and predict relationships. The bulk and tapped densities were determined and the volume as a fiinction of the number of taps was evaluated to give the values for angle of internal flow and the compaction constant T. Carr's compressibility index was calculated from the values of the bulk and tapped densities. Further powder investigations included shear testing. The powders were filled into hard gelatine capsules (size 1) on an instrumented Bosch GKF-400S tamp filling machine with a 19.6mm dosing disk. The capsules were filled at different compression settings. For the theophylline study the tamping force was measured at tamping stations 3 and 4 and for the ibuprofen study two powder bed heights were employed. The optimal filling performance was evaluated by the fill weight, coefficient of fill weight variation, tamping force and density of capsule plug. Disintegration and dissolution tests were also carried out. These powder formulations containing the poorly flowing drug theophylline or the low melting point drug ibuprofen could be satisfactorily filled on the tamp filling capsule machine. For the theophylline study, tamping forces were always greater at tamping station 3 than at station 4, indicating that the capsules were mainly full by the time they reached the third station. The increased powder bed height for ibuprofen slightly increased the capsule fill weight and decreased the coefficient of fill weight variation. It was found that higher compression settings were associated with higher fill weights and lower coefficients of fill weight variation. The flow properties of the powder also influenced the filling of powders into capsules. By altering the concentration of the filler, it was possible to change the flow properties, which further affected the filling performance of the powder mixtures. The type and level of excipients influenced the disintegration and dissolution properties of the powder blend.