Abstract Synthesis of fluid inclusions in the CH 4H 2ONaCl system was accomplished by subjecting fractured quartz, along with known quantities of CH 4, H 2O, and NaCl, to a pressure of 1 kbar and temperatures of 400, 500, or 600°C, in sealed Au capsules. Under the elevated P−T conditions some of the fractures healed, trapping fluids as inclusions. Microthermometric measurements conducted on the fluid inclusions show that at 1 kbar and 400 to 600°C, there are very broad regions of fluid unmixing in the CH 4H 2ONaCl system. For those bulk fluid compositions that lie in the two-phase (i.e., immiscible fluids) field, the high density phase is enriched in NaCl, whereas the low density phase is enriched in CH 4. For any given bulk composition, the degree of NaCl enrichment in the high density phase increases, whereas the degree of CH4 enrichment in the low density phase decreases, as temperature increases from 4.00 to 600°C.