This report proposes a properdesigned method for the RAD7 detector to measure 222Rn in gases discharged from sulfurous fumaroles, tested at temperatures between 94°C and 162°C. RAD7 is one of the most widely used commercial instruments for 222Rn measurements, either dissolved in water or fluxing from soils, offering a reliable comparison of data acquired in different laboratories being already calibrated by the manufacturer. However, RAD7 cannot be used for measurements in hot and acidic gas soils. The properdesigned method involves sampling of the gas insitu in Tedlar® bags, transferring the gas into glass tubes and, finally, measuring the gas via a closed loop using the RAD7 after the removal of H2S by proper traps. This new method has been tested for measuring radon in the hostile environments of Solfatara and Pisciarelli in the Campi Flegrei volcano (Italy), providing the instrumental base for creating timely Rn content datasets.