Crystals of disodium succinate have two distinct modifications, monoclinic and triclinic. These were recognized by electron spin resonance (e.s.r.) studies of. the free radicals produced by γ-irradiation. In the monoclinic crystals, e.s.r. spectra and radiation processes appear similar to those observed in γ- (or x-) irradiated succinic acid. The situation in triclinic crystals is more complex (see Part 2). In monoclinic crystals irradiated at 77 °K, two types of radical coexist. They were identified as −O2CCH2CH2ĊO22−, 1, together with a distorted conformation of the radical −O2C(ĊHCH2)*CO2−, 2. On warming to room temperature the former species disappears while the latter changes irreversibly to a stable conformation −O2CĊHCH2CO2−, 3.