The effect of noncompacted and compacted hydrophilic as well as hydrophobic colloidal silicon dioxide (CSD) on tableting properties of three different pharmaceutical excipients used for direct compression, namely, Avicel PH 101, Starch 1500, and Tablettose 80, was investigated. Binary powder mixtures containing 0.5% CSD and 99.5% excipient were compressed on an instrumented single-punch tablet press, and the radial tensile strength/compaction load profiles were examined. The Ryshkewitch-Duckworth relationship shows that the influence of CSD on tablet strength was dependent on the hydrophobic and hydrophilic nature of the CSD and on the compaction characteristics of the excipients. Tablets from each excipient with and without CSDs were subjected to different levels of relative humidity at 20 degrees C for 7 days. The sorption isotherms and the radial tensile strengths of the tablets after the storage period showed that neither hydrophilic nor hydrophobic CSD influenced the tablet properties of Avicel PH 101, Starch 1500, and Tablettose 80. Moreover, ternary powder mixtures containing magnesium stearate as a third component were compressed in order to study the influence of CSD on the deleterious effect of magnesium stearate on the interparticle bonding. The radial tensile strength/compaction load profiles and the residual and ejection forces of tablets made from ternary mixtures showed that CSD eliminated the negative effect of magnesium stearate on interparticle bonding while maintaining the lubrication action, in a manner that was affected by its hydrophobicity/hydrophilicity and by the particle deformation properties of the excipient upon compression.