The mapping of CO2 soil diffuse degassing was carried out in the Pico Vermelho geothermal area, located in the northern flank of Fogo Volcanoin S. Miguel Island. Fieldwork took place between September and November 2002 and CO2 concentrations were determined according to the methodology proposed by Baubron et al. (1994) and Ferreira and Gaspar (1997), using
a portable CO2 infrared detector with 0,1 vol.% accuracy. Soil temperature was also measured and a thermal anomaly map was produced for the same area. Maximum CO2 soil concentration reached 96,6 vol.% and soil temperature values ranged from 13,6 °C to 98,6 °C. The soil diffuse degassing area exhibits a spatial development trending NW-SE, which is parallel to the main structural direction in the region well defined by the Ribeira Grande Graben. CO2 and temperature anomalies are coincident in space showing a main N-S elongation where values are the highest. The existence of NW-SE and NE-SW sharp limits in both anomalies is thought to be due to the presence of hidden faults with that direction.