Factor apalysis is applied to results of chemical analyses of 103 water samples from wells in the Upper and Middle Mojave River valley, San Bernardino County, California. Chemical analyses showed that there are three principal chemical types of water, calcium bicarbonate, sodium sulfate, and sodium chloride, as well as many mixtures of the three. Data were studied by factor analysis to learn the relative importance of each principal ion in determining the variations among the samples, and to examine the possibility of chemical equilibrium between aqueous and solid phases in the aquifers.Most of the covariance in the system may be accounted for by variances of Ca +•, Mg +•, Na +•, SO• -•, and C1-L There is almost identical loading on the constituents Na +• and C1-L The variance in chemical composition of the hydrochemical system is governed largely by sources of sodium chloride. None of the components is controlled by equilibrium between ions in the water and minerals in the aquifers. Concentrations of NO8 -• and F -• vary independently of other constituents. Geographic distribution of statistical loadings of the principal constituents at individual wells does not reveal sources of the constituents, which must be deduced from geologic and hydrologic evidence. Factor analysis, however, furnished the critical information on chemical relationships basic to the deduction. (GEOLOGIC AND HYDROLOGIC ENVIRONblENT The Upper Mojave River valley occupies about 650 square miles near the southwestern apex of the Mojave Desert (Figure 1) and is defined [Bader et al., 1958] as that part of the river valley extending from the base of the San Bernardino Mountains to the river narrows near Victorville, about 20 miles downstream. In this area, most water samples are distinct as to chemical type: calcium bicarbonate, sodium sulfate, or sodium chloride. About 400 square miles of the Middle Mojave River valley [Page et al., 1960] adjacent to the upper valley is included in the present study. There the waters are mostly varied mixtures of the three original chemical types. The Mojave River is formed by the junction of the West Fork Mojave River and Deep Creek. These streams rise on the north side of the San Bernardino Mountains, near Cajon Pass. The rocks of the mountains are mostly metamorphic and igneous [Dibblee, 1957], but toward the west include sedimentary types. The metamorphic terrane is a complex of gneiss, biotite schist, mica-quartz-albite schist, quartzite, and marble of Precambrian (?) age intruded by quartz diorite, granodiorite, and diorite of Mesozoic age or older. Some of the schist is pyritic (L. C. DutcheL oral communication, 1965). The igneous terrane consists mostly of biotite quartz monzonite but includes granite and granodiorite. It is probably of late Jurassic or early Cretaceous age, but in part may be older. The sedimentary rocks in and north of the drainage basin of West Fork Mojave River are continental deposits of sandstone, siltstone, and shale, which locally include some conglomerate, limestone, dolomite, ...