The primary purpose of this report is to present all chemical data from the Salton Sea area collected by the U.S. Geological Survey between 1995 and 2001. The data were collected primarily for the Department of the Interior's National Irrigation Water Quality Program (NIWQP). The report also contains a brief summary and citation to investigations done for the NIWQP between 1992 and 1995. The NIWQP began studies in the Salton Sea area in 1986 to evaluate effects on the environment from potential toxins, especially selenium, in irrigation-induced drainage. This data report is a companion to several reports published from the earlier studies and to interpretive publications that make use of historical and recent data from this area. Data reported herein are from five collection studies. Water, bottom material, and suspended sediment collected in 1995-96 from the New River, the lower Colorado River, and the All-American Canal were analyzed for elements, semi-volatile (extractable) organic compounds, and organochlorine compounds. Sufficient suspended sediment for chemical analyses was obtained by tangential-flow filtration. A grab sample of surficial bottom sediment collected from near the deepest part of the Salton Sea in 1996 was analyzed for 44 elements and organic and inorganic carbon. High selenium concentration confirmed the effective transfer (sequestration) of selenium into the bottom sediment. Similar grab samples were collected 2 years later (1998) from 11 locations in the Salton Sea and analyzed for elements, as before, and also for nutrients, organochlorine compounds, and polycyclic aromatic hydrocarbons. Nutrients were measured in bottom water, and water-column profiles were obtained for pH, conductance, temperature, and dissolved oxygen. Element and nutrient concentrations were obtained in 1999 from cores at 2 of the above 11 sites, in the north subbasin of the Salton Sea. The most-recent study reported herein was done in 2001 and contains element data on suspended material isolated by continuous-flow centrifugation on samples collected in transects extending out from the Whitewater, the Alamo, and the New Rivers into the Salton Sea. Chemical data on suspended sediment and bottom material from tributory rivers and the Salton Sea itself show that many insoluble constituents, including selenium and DDE, are concentrated in the fine-grained, organic- and carbonate-rich bottom sediment from deep areas near the center of the Salton Sea. Data also show that selenium and arsenic are markedly enriched in seston (plankton, partially-degraded algal detritus, and mineral matter that compose suspended particulates in the lake) collected just below the water surface in the Salton Sea. This result indicates that bio-concentration in primary producers in the water column provides an important pathway whereby high selenium residues accumulate in fish and fish-eating birds at the Salton Sea.