New derivatives of isoindole -4,7-dione have been synthesized and their radiation sensitization and chemical behavior have been studied. One-electron reduction potentials have been determined by pulse radiolysis and found to be in the range of -0.45 to -0.36 V vs NHE . Radiosensitization effects were tested in vivo using soft tissue sarcoma transplanted in mice. All the isoindole -4,7-diones tested were found to exert considerable radiosensitization, approaching that of misonidazole tested under comparable conditions. The derivatives which contain a carbethoxy group on the pyrrolic ring were found to have more positive reduction potentials and to act as more efficient sensitizers. Further development of this class of radiosensitizers is underway.