Stability tests have been performed on two typical iodine-containing radiopaque poly(methacrylate) copolymers. Material A is a terpolymer of methylmethacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA) and 2-[4-iodobenzoyl]-oxo-ethylmethacrylate (4-IEMA); material B is a copolymer of MMA and 4-IEMA. Cylindrical specimens of material A were implanted subcutaneously and intraperitoneally in Wistar rats. The implants were retrieved after 2 years. Histology showed that the material was well-tolerated. Detailed analysis of the surface of the implants by electron spectroscopy for chemical analysis (ESCA) revealed that the material remained stable. No differences could be detected between the ESCA spectra of the explants, and those of the control specimens, which were from the same synthetic batch and which were stored in dry form during the entire experimental period. Material B was also stable upon irradiation with X-rays in vitro, even at high doses, compared to the clinical situation. Exposure of material B to γ-radiation, however, was found to lead to structural degradation. This was evident from clear yellowing, and also from the ESCA spectra. The spectra revealed that material B deteriorates during γ-irradiation through rupture of C–C and/or C–O chemical bonds, not via C–I bond disruption. It can be concluded that iodine is tightly bound to these radiopaque biomaterials. This is important with regard to potential applications of these materials as implant biomaterials.