Ilopango Caldera in central El Salvador is filled by Ilopango Lake. In November and December of 1999, radon, thoron, carbon dioxide, and mercury soil gas concentrations were obtained at 106 points within the caldera, as well as carbon dioxide efflux. The spatial distribution of the concentrations of these gases and carbon dioxide efflux show that the values of Ilopango Caldera are within background levels of other active volcanoes of the world and El Salvador. However, several areas where anomalies of high radon, thoron, carbon dioxide, and carbon dioxide efflux coincide were identified in regions of dense faulting. Heavier carbon isotope values between −13‰ and −20‰ (overall range: −13.1‰ to −29.8‰) in the anomalous regions suggest mixing of minor amounts of volcanic gases with biogenic gases or the presence of C₄ plants. Degassing of carbon dioxide from the lake was calculated using a double boundary layer model. A total carbon dioxide efflux of 644 to 1111 t d⁻¹ was calculated for the lake and the soils of the caldera. Considering the low soil degassing of carbon dioxide (0.3–3.9 g m⁻² d⁻¹), radon (1.2–108.5 pCi L⁻¹), and mercury (0–0.016 mg m⁻³), Ilopango can be considered a quiescent caldera at the time of this survey.