Irradiation of a single crystal of Ph₃As⁺ CH₃ I^- produces a paramagnetic species exhibiting hyperfine interaction with ⁷⁵As and methyl protons. By studying the angular dependence of the E.S.R. spectra obtained from X-irradiated crystals of Ph₃As⁺ CD₃I^- and Ph₃As^{+ 13}CH₃I^- the following tensors have been obtained : g, quadrupolar interaction with ⁷⁵As, magnetic hyperfine coupling with ⁷⁵As and ¹³C. These parameters show that ~ 75 per cent of the spin is localized on the methyl carbon and lead to the identification of the novel adduct Ph₃As ... CH₃. The directions of the eigenvectors show that the C₃ axes of Ph₃As and CH₃ are probably not colinear. The structure of the adduct is compared to that of phosphoranyl radicals.