Abstract: Scintillation cells are used typically for measuring the concentration of 222Rn in air and are calibrated for that purpose. However, scintillation cells are sometimes used for measuring 222Rn in natural gas or carbon dioxide. The counting efficiencies of scintillation cells for measurements of 222Rn in these gases should be different from those for measuring 222Rn in air because the ranges of alpha particles emitted by 222Rn and its progeny are greater in methane and smaller in carbon dioxide than in air. If these effects are not taken into consideration, measurements of 222Rn in natural gas will be biased high and in carbon dioxide will be biased low. The authors previously investigated the effects of barometric pressure on measurements of 222Rn in air using scintillation cells. A modeling technique was used in a previous study to calculate theoretical errors that would result if atmospheric pressure were not considered. In the current study, the same modeling technique was used to calculate theoretical errors that would be made for measurements of 222Rn in methane and carbon dioxide if the calibration for 222Rn in air were used. Results are presented for four types of scintillation cells of varying geometries and for barometric pressures representative of four elevations ranging from sea level to 1,963 m (6,440 feet). These results indicate that the errors introduced by the ranges of the alpha particles in gases different from air can be significant. Depending on the type of cell and the local pressure, a measurement of 222Rn in methane may be biased high by 2–7%, while a measurement of 222Rn in CO2 may be biased low by 15–20% if the calibration for 222Rn in air is used.