The scientific activity has been always dedicated to the geochemistry of fluids. From 1979 to 1988, the research has concerned mainly theoretical and applied aspects of the geochemistry of geothermal systems. From 1988 to present, the activity regarded mainly the geochemistry of volcanic fluids finalised to the volcanic surveillance and the estimation and interpretation of the CO₂ earth degassing. Both the diffuse gas flux from the soil that affects volcanic and geothermal areas, and the flux of deeply derived CO₂ that interests wide areas forming regional anomalies, have been investigated. In particular, Chiodini participated to the development of the methodology and of the instrument for quick measures of the CO₂ flux generated by diffuse degassing of the soil. The method is now used by many researchers worldwide both for geothermal investigations and volcanic surveillance. The regional CO₂ Earth degassing has been investigated by applying mass budget of dissolved carbon in groundwaters using stable isotopes. This method has been used for the realization of the map of CO₂ fluxes interesting the whole Apennine area of central Italy. This map constitutes the first (and probably unique) example in the world of measure of the  earth degassing' from a wide area. Other researches regarded the geochemistry of natural gas emissions associated with volcanoes and geothermal system. The gas equilibria in the H₂O-H₂-CO₂-CH₄-CO gas system were investigated in order to derive gas-geoindicators of temperature-pressure and redox conditions affecting hydrothermal-geothermal systems. An other research was focussed on the origin of the fluids inferred from stable isotopes highlighting the occurrence of fractionation processes related to the oxygen exchange between the CO₂ and H₂O molecules in fumarolic fluids. The research activity regarded different worldwide volcanoes and geothermal systems including: Campi Flegrei caldera, Vesuvio, Vulcano, Latera caldera, Pantelleria (Italy); Teide (Canary Islands, Spain); Nisyros (Greece); Yellowstone, Long Valley caldera (USA); Domuyo, Peteroa and Copahue volcanoes (Argentina); Guagua Pichincha and Quiulotoa (Equador); Furnas, Azores ​(Portugal).