Water–rock interaction experiments have been performed to investigate the production of CO2-rich gas in the deep reservoir of the geothermal system of Mt Amiata (Italy) through the interaction between a natural phyllite sample (coming from 1115 m below ground level and considered representative of the deep reservoir) and a coexisting fluid phase. Experiments were conducted with an externally heated pressure vessel, a chamber furnace and a stirred micro-reactor at 300 °C and at P up to 35 MPa, using different solid/liquid ratios, and also under nominally anhydrous conditions. Experimental results show that CO2-rich gases containing variable amount of CO, CH4 and other aliphatic and aromatic hydrocarbons were produced by fluid–rock interaction. Compositional data of the gas, coupled with solid residua characterization, show that thermal decomposition of magnesian siderite in phyllite is the process responsible for the gas genesis. This process is particularly effective when aqueous solutions interact with phyllite and it may represent the thermometamorphic reaction mainly responsible of the production of CO2-rich gas under hydrothermal conditions similar to those of the main reservoir in the Mt Amiata geothermal area.