Gas chemical and isotopic compositions are reported for samples collected from 9 volcanic and geothermal areas in California: Medicine Lake Volcano, Mount Shasta, Lassen Volcanic Center, Clear Lake Volcanic Field, Mono Lake Volcanic Field, Long Valley Caldera, Mammoth Mountain, Coso Volcanic Field, and Salton Sea Geothermal Field. Beginning in 2014, gas samples were collected as part of the USGS California Volcano Observatory's newly established gas geochemical monitoring program for moderate to very-high-threat volcanoes. Data on gas samples collected prior to 2014 by this and previously published studies are also reported to provide longer-term records for some areas. Gas samples in this study were analyzed for chemical (CO2, H2S, He, H2, Ar, O2, N2, NH3, CH4, C2H6, CO, C3H8, and C4H10) and isotopic (R/RA, d13C-CO2, d13C-CH4, dD-steam, d18O-steam) compositions at USGS laboratories in Menlo Park, California, Denver, Colorado, and Reston, Virginia. The chemical and isotopic data are saved in three spreadsheets in the *.csv format.