Radiolytic particle formation - the formation of aerosol particles in initially particle-free air through irradiation with ionizing radiation - can be a useful process for the detection of gaseous contaminants in clean air, or an undesirable source of particles in experimental setups. In this work, we observed radiolytic particle formation inside a soft x-ray irradiation chamber in filtered ambient air, but also in active-carbon-filtered synthetic air and nitrogen (> 99.999% purity). The largest of the radiolytic particles grew to several 100 nm in diameter. A single-particle aerosol mass spectrometer ( LAAPTOF) was used to conduct the first direct measurement of the chemical composition of individual radiolytic particles. Total sample amounts as small as 0.7 ng were analyzed. The mass spectra suggest a chemical composition similar to that of ambient aerosols, with typical secondary species such as nitrate, sulfate, and organics. The strong presence of nitrates is in agreement with previous conclusions on the chemical composition of radiolytic particles, while the presence of organics is shown in this work for the first time. The results show that even very low levels of gaseous organic contaminants in high-purity gases can be detected by virtue of radiolytic particle detection, and that impurities in manufactured gases must be carefully controlled for in setups where radiolytic particle formation would cause measurement artefacts.