We investigated the effect of radon (222Rn) contamination of samples and chemicals used for sample preparation and gas purification on the 14C age of samples. Radon count rate was monitored over several 222Rn half-lives in a proportional counter filled with methane. We analyzed ca. 240 samples of fossil and recent wood, charcoal, bone, dissolved inorganic carbon in water, travertine, peat, aquatic plants, organic soil and atmospheric CO2 for 222Rn contamination. Chemicals used for gas purification (MnO2, Mg(ClO4)2), ruthenium-coated pellets used for catalytic reduction of CO2 to CH4, and acids used for dissolution of calcareous samples (HCl and H3PO4) are also potential sources of Rn. Most geological samples contained a significant amount of Rn, as opposed to samples of wood, charcoal, aquatic plants and atmospheric CO2. We also studied Rn contamination of water samples during tritium activity measurements in a CH4-filled gas proportional counter. We found an increased count rate in the 3H channel, as well as above the 20 keV region in Rn-contaminated groundwater samples.