The effect of ambient atmospheric conditions during x‐ray exposure on the sensitivity of a new type positive resist, which contains poly(2‐methylpentene‐1‐sulfone) as the radiation‐sensitive compound, is studied. Resist sensitivity to x rays is seen to depend strongly on oxygen pressure during exposure. Sensitivity in air is only one‐sixth of that in a vacuum. Using Fourier transform infrared spectral analysis of the resist film following x‐ray irradiation, it is found that the alkyl radical of poly(2‐methylpentene‐1‐sulfone) generated by x‐ray irradiation is easily scavenged by the oxygen molecule in air, thus retarding its further unzipping decomposition.