The Ansanto Valley (southern Italy) is characterized by vents and boiling mud lakes that emit typical volcanic exhalations (mostly fossil CO₂). This fossil dilution spreads over the Ansanto Valley and its impact on local trees is investigated in this study. Six trees at increasing distance from the emitting sources and 2 aliquots of gas were sampled. Dendrochronological analysis was performed on tree cores in order to check the accuracy of the tree-ring sequences; the results indicate no anomalies in the curves of the analyzed trees. δ¹³C and radiocarbon (¹⁴C) analyses were performed on the α-cellulose extracted from some selected tree rings. The main aim of δ¹³C analysis was to gain information about the origin of CO₂ arising from the source; the results support the hypothesis of a carbonatic origin, with respect to a volcanic origin. ¹⁴C analysis was performed to evaluate the influence and to quantify the percentage of fossil dilution characterizing the local atmosphere and affecting the trees at different distances from the source during the years. The results show the presence of a strong fossil dilution affecting the trees, increasing toward the sources (from ∼6% at 80 m distance to ∼30% at 20 m from the nearest vent) with quite stable values over the examined period.