Research at Yucca Mountain in southern Nevada (Fig. 1), is being supported by the US Department of Energy to evaluate this site as a possible high-level radioactive waste repository. Yucca Mountain is underlain by a thick sequence of ash-flow and bedded tuffs, with a few silicic lavas.Variations in mode of tuff emplacement and postemplacement alterations have given rise to pyroclastic rocks of quite variable character, ranging from nonwelded to densely welded, vitric to devitrified, and nonzeolitized to completely zeolitized. The proposed repository horizon is in the lower portion of the thick, densely welded Topopah Spring Member of the Paintbrush Tuff in the unsaturated zone. Within the Topopah Spring Member and in the rocks beneath the proposed repository horizon, there are significant variations in mineralogy . Such changes in mineralogy include the localized occurrence of such potentially reactive phases as cristobalite,tridymite, smectite, and volcanic glass. The important sorptive minerals clinoptilolite and mordenite also occur in discrete horizons, and their distribution changes horizontally and vertically. We have undertaken a study of the mineralogy in Yucca Mountain as a function of depth and lateral position to predict the horizontal and vertical distribution of these important potentially reactive and sorptive minerals. This knowledge has aided in locating the repository horizon and will help to put bounds on mineralogic variability within the repository horizon. In addition, studies of the distribution of minerals in Yucca Mountain allow us to deduce the factors that have controlled mineral distributions and to predict mineral assemblages along transport pathways [1,2]. In addition, we are investigating the groundwater chemistry because it and mineralogy are used as input to codes for calculating the transport rate of waste elements from the repository to the accessible environment.