Abstract: A viscometric study of dilute montmorillonite dispersions, truing for the most part dilution-type capillary viscometers supplemented by a falling-head capillary viscometer, has involved a determination of the concentration dependence of viscosity of such dispersions in both distilled and electrolyte-containing water. The results of the study have been interpreted in terms of a theory of incremental viscosity, and of the Schulz-Blaschke (1941) equation. Use of this equation yields good extrapolations to zero concentration for the determination of limiting viscosity number, and permits calculation of “interaction indices” which are considered, for reasons discussed in detail, to be a measure of particle-particle interaction. These indices are related to the cationic heat of hydration of the electrolyte used, and relationship is demonstrated between the yield stress of a 3 percent montmorillonite-electrolyte-water system and such interaction indices. The recent work of Packter (1956, 1957) is discussed, and the relation of the results of this investigation to the data of van Olphen (1956, p. 204) is noted.