Fluids have been sampled from 9 wells and 2 fumaroles from the East Flank of the Coso hydrothermal system with a view to identifying, if possible, the location and characteristics of the heat source inflows into this portion of the geothermal field. Preliminary results show that there has been extensive vapor loss in the system, most probably in response to production. Wells 38A-9, 51-16 and 83A-16 show the highest CO2-CO-CH4-H2 chemical equilibration temperatures, ranging between 300-340oC, and apart from 38A-9, the values are generally in accordance with the measured temperatures in the wells. Calculated temperatures for the fractionation of 13C between CO2 and CH4 are in excess of 400oC in fluids from wells 38A-9, 64-16-RD2 and 51A-16, obviously pointing to equilibrium conditions from deeper portions of the reservoir. Given that the predominant reservoir rock lithologies in the Coso system are relatively silicic (granitic to dioritic), the isotopic signatures appear to reflect convective circulation and equilibration within rocks close to the plastic-brittle transition. 3He/4He signatures, in conjunction with relative volatile abundances in the Coso fluids, point to a possibly altered mantle source for the heat source fluids.