Carbon and oxygen isotope analyses have been performed on calcium carbonate samples from flysch rocks collected in the south of Tolfa and Allumiere villages (Central Italy). Large variation in delta <sup>18</sup> O occurs from values around +28 per mil (SMOW), typical of marine carbonates, down to values of about +9 per mil (SMOW). The lower values are measured on samples collected close to phanerocrystalline carbonate deposits outcropping in the area and interpreted as part of a hydrothermal vein system. delta <sup>13</sup> C does not show large variation, and, aside from a few outliers, falls within a range of 0 and +2 per mil (PDB). Similar carbon and oxygen isotope spatial relationships between the flysch country rocks and the phanerocrystalline carbonate deposits were determined in a previous study by Ferrini & Masi (1987), who interpreted the <sup>18</sup> O depletion as due to interaction of the host rock with an aqueous fluid at hydrothermal temperatures below 250 degrees C and with a water to rock ratio progressively increasing toward the carbonate vein. The combined results indicate that hydrothermal fluid temperature largely regulates oxygen isotope variation (and possibly the carbon and oxygen deltas co-variation) with respect to the variation of the water to rock ratio.