The refractive index of bulk poly(methyl methacrylate) samples is modified by X-ray exposure. The intensity of the radiation is distributed over an energy range of 1 to 20 keV with a maximum at 3 keV. For an absorbed dose of about 1 kJ cm−3, an increase of the refractive index of up to 3×10−4is observed. However, for much lower doses of about 10 J cm−3a decrease of 4×10−4appears. These refractive index changes, as well as the simultaneously arising radiation-induced thickness changes, are detected interferometrically. Based on these observations, on dose calculations, and on literature data, a model is proposed which describes quantitatively the observed refractive index changes. The refractive index increase is caused by a radiation-induced chemical alteration of the polymer composition, while the refractive index decrease is correlated with a decrease of mass density.