Chemical analyses of water from 99 wells and 19 springs in the San Luis basin in Colorado and New Mexico were evaluated to determine selected water quality characteristics as an aid in understanding the flow of groundwater in the basin. The evaluation shows that the distribution of chemical water types in the basin is consistent with chemical changes to be expected along flow paths in rocks typical of those in the basin. The San Luis basin area is underlain by a surficial (< 100-ft-thick) unconfined aquifer and, in turn, by a confining bed and a deeper confined aquifer. Previous studies have indicated that the groundwater system is recharged around the edges of the basin and that groundwater then moves toward discharge areas in the topographically closed part of the basin and along principal streams. The evaluation of water quality data showed that groundwater at the perimeter of the San Luis basin is primarily a calcium bicarbonate type, which is typical in recharge areas. Groundwater near the center of the basin is primarily a sodium bicarbonate type, which is typical of groundwater in downgradient areas. The change in principal cation from calcium to sodium indicates chemical evolution of the water along the groundwater flow path and supports previously developed concepts of groundwater movement in the basin. The exchange of calcium for sodium along the flow path also is assumed to occur in the Conejos River subbasin. Upgradient wells yield calcium bicarbonate type water, whereas downgradient wells yield sodium bicarbonate type water. However, an exception to this relation is found at McIntire Spring, which yields calcium bicarbonate type water from a downgradient location. The source of water discharging from the spring may be the confined aquifer, with hydraulic connection along the Manassa fault. The concentration of dissolved solids in water from both the unconfined and confined aquifers increases downgradient. The increase is dramatic in the closed basin, ranging from < 500 mg/L to > 30 ,000 mg/L. In this area, the normal increase in concentration by dissolution of minerals along the flow path is exceeded by the increase due to evapotranspiration from the shallow water table. (Lantz-PTT)