Abstract Poly(methyl methacrylate) is a preferred material used in X‐ray lithography. One effect observed after irradiation is a change of its chemical structure, combined with a decrease of the molar mass. This was analysed quantitatively using size exclusion chromatography combined with multi‐angle laser‐light scattering. Based on a kinetic model, an equation was derived which gives a satisfactory description of the dependence of the molar mass on the radiation dose.