X-Irradiation of single crystals of methylene diarsonic acid gives rise to a variety of radicals which have been identified by ESR. spectroscopy: (OH)₂As(O)CHAs(0) (OH)_2, -OAs(OH)₂CH₂As(O)(OH)_2, AsO₃^-- and (OH)As (O)CH₂As(O)(OH)₂. The spin densities have been obtained from the ⁷⁵As-hyperfine tensors. An additional species - probably a radical anion localized on a pentacoordinated arsenic - is also detected after annealing at room temperature. The radiation damage process is discussed.