We report the first detailed study of spatial and temporal variations on the diffuse emission of carbon dioxide from the Santa Ana–Izalco–Coatepeque volcanic complex. Soil CO₂ efflux measurements were performed at 447 sampling sites and reached values up to 293 g m⁻² d⁻¹ for the March 2001 survey. Most of the diffuse CO₂ degassing is occurring at the center of this volcanic complex, near the Cerro Pacho dome where the intersection of one of the regional NW-SE fault/fracture systems and the south Coatepeque caldera rim occurs. Soil CO₂ efflux measurements were not performed inside Santa Ana's summit crater due to accessibility problems at the time of this survey. The total diffuse CO₂ emission for this volcanic complex was estimated at ∼600 t/d. Low CO₂ efflux values were identified on the flanks and summit regions of the Izalco and Santa Ana volcanoes. Isotopic data of gases collected from low-temperature fumaroles in the study area showed ³He/⁴He ratios close to the atmospheric composition, except for the fumarolic discharges inside Santa Ana summit crater (7.54). Carbon isotopic signature of these gases suggested that 74% of CO₂ is limestone-derived from thermodecarbonation processes, while the fumarole sample from Santa Ana crater showed ∼48% of limestone-derived CO₂. Temporal variations of the diffuse CO₂ degassing observed at the Cerro Pacho dome ranged from 4.3 to 327 g m⁻² d⁻¹, with a median value of 98 g m⁻² d⁻¹. Time domain analysis of the soil CO₂ efflux showed a strong autoregressive behavior, whose covariance slowly dies through time. Spectral analysis showed the existence of soil CO₂ efflux variations at diurnal and semidiurnal frequencies. This finding suggests that soil CO₂ efflux variations are coupled with those of meteorological variables (i.e., barometric pressure, wind speed, etc.), accounting for much of the behavior of the soil CO₂ efflux time series at the short-term scale.