We have measured viscosities and densities of dilute aqueous suspensions of Na-kaolinite (25 °C), of Na-montmorillonite (25–55 °C), and of mixtures of Na-kaolinite/Na-montmorillonite (25 °C). Results of the measurements at 25 °C have been analyzed in terms of the Einstein equation with conclusions that are consistent with previous knowledge that colloidal particles of these clays are non-spherical and that montmorillonite is a swelling clay. Viscosities of mixed clay systems are nearly consistent with a simple additivity model; the small non-additivities are in accord with previous evidence that relatively small montmorillonite particles are adsorbed on the surface of relatively large kaolinite particles. The effect of temperature on the viscosities of suspensions of montmorillonite is consistent with the idea that aggregation of clay particles diminishes with increasing temperature.