| Summary: | The study concerns the analysis of <sup>220</sup>Rn (thoron) recorded in the surface soil in two sites of the Campi Flegrei caldera (Naples, Southern Italy) characterized by phases of volcanic unrest in the seven-year period 1 July 2011–31 December 2017. Thoron comes only from the most surface layer, so the characteristics of its time series are strictly connected to the shallow phenomena, which can also act at a distance from the measuring point in these particular areas. Since we measured <sup>220</sup>Rn in parallel with <sup>222</sup>Rn (radon), we found that by using the same analysis applied to radon, we obtained interesting information. While knowing the limits of this radioisotope well, we highlight only the particular characteristics of the emissions of thoron in the surface soil. Here, we show that it also shows some clear features found in the radon signal, such as anomalies and signal trends. Consequently, we provide good evidence that, in spite of the very short life of <sup>220</sup>Rn compared to <sup>222</sup>Rn, both are related to the carrier effect of CO<sub>2</sub>, which has significantly increased in the last few years within the caldera. The hydrothermal alterations, induced by the increase in temperature and pressure of the caldera system, occur in the surface soils and significantly influence thoron’s power of exhalation from the surface layer. The effects on the surface thoron are reflected in both sites, but with less intensity, the same behavior of <sup>222</sup>Rn following the increasing movements and fluctuations of the geophysical and geochemical parameters (CO<sub>2</sub> flux, fumarolic tremor, background seismicity, soil deformation). An overall linear correlation was found between the <sup>222−220</sup>Rn signals, indicating the effect of the CO<sub>2</sub> vector. The overall results represent a significant step forward in the use and interpretation of the thoron signal.
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