An ion‐chamber gas mixture having an atomic composition similar to that of A150 plastic has been tested experimentally. This “A150 gas”, air, and methane‐based TE gas were each flowed through A150 plastic‐walled ion chambers of various sizes, and irradiated with60CO γ rays and 14.8‐MeV neutrons, separately administered. Observed ionization was measured and corrected for recombination losses. Best values forW̄A150, averaged over all chamber sizes, were 26.2 eV/ion pair for60CO γ rays and 28.2 eV/ion pair for 14.8‐MeV neutrons.