The hydrothermal system beneath Campi Flegrei, an explosive caldera in southern Italy, is quantified by analyzing gas samples collected since 1983, during and after 2 years of intense earthquake swarms and dramatic uplift of the caldera center. These gas samples were obtained from both land and submarine fumaroles, as well as a few samples from water wells. At the Bocca Grande fumarole in Solfatara Crater, near the center of Campi Flegrei, large variations were measured in the amount of water vapor, hydrogen sulfide, nitrogen and hydrogen gases, methane, and hydrogen chloride. Some variations could be controlled partly by a change in heat flux from a deep source to a surficial hydrothermal system. Possible evidence of a change in heat flux comes from subsurface temperatures, calculated from a water-carbon monoxide reaction. The calculated temperatures changed most in 1989-1990. Concurrent changes were measured in concentrations of hydrogen sulfide, nitrogen, and hydrogen. No change was noted in helium or carbon isotopes, which could have indicated a change in emission of magmatic gases. I suggest the changes in gas chemistry since 1985 could be evidence of an aborted period of unrest, which was characterized as a change in gas emission but did not progress far enough to produce uplift or earthquake swarms. As pointed out by other researchers, the ratio of helium isotopes, which implies the presence of magma, is uniform across an area that includes Campi Flegrei. The distribution of carbon isotopes, which is nonuniform, correlates with limestone outcrops and the presence of shallow thermal sources. ¿ American Geophysical Union 1994