Polyimide films and glass fiber reinforced plastics (GFRPs) are used as insulating and structural materials for superconducting magnets in a neutrino beam line of the Japan Proton Accelerator Research Complex (J‐PARC). It is indispensable to evaluate radiation resistance of these materials, because they are required to keep their electrical and mechanical properties in a high radiation field of 30 kGy/year at low temperature of 4 K. In this work, the gas evolution resulting from chemical reactions induced in the polyimide films and GFRPs by γ‐ray irradiation at 77 K was investigated. It was found that the main component of the evolved gas was hydrogen. The amount of hydrogen evolved from the superconducting magnet system used in the neutrino beam line was estimated to be 0.37 mol/year corresponding to 0.01 L/year as the volume of liquid hydrogen. It indicates that the hydrogen evolved from the organic materials will not influence the operation of the helium purifying system. © 2006 American Institute of Physics