The infrared spectra of mulls of the trimethylarsenic dihalides, (CH 3) 3AsX 2, are reported and assignments have been made by comparing these spectra with those of the deuterated analogs, (CD 3) 3AsCl 2 and (CD 3) 3AsBr 2. These compounds have a pentacovalent trigonal bipyramidal structure in the solid state when X is F or Cl, but have an ionic structure with tetracovalent arsenic a halide ion when X is Br or I. 1H and 19F nmr spectra are reported for solutions of (CH 3) 3AsF 2 and (CH 3) 3AsCl 2 and are compatible with trigonal bipyramidal geometry. Triphenylarsenic difluoride, on the basis of the IR and 19F nmr spectra, is a pentacovalent trigonal bipyramidal molecule, but the structures of the corresponding chloride and bromide could not be unequivocally determined from their IR spectra. previously reported for these compounds by other workers. The discrepancy arises because even short exposure of triphenylarsenic dichloride or dibromide mulls to the atmosphere leads to the development of absorption peaks in the IR spectra which are characteristic of the triphenylarsenic hydroxyhalides. The IR spectra of pure samples of several trimethyl- and triphenylarsenic hydroxy compounds, R 3AsOHY, were determined and compared with the spectrum of the deuterated analog R 3AsODY.