Abstract The proton and carbon-13 NMR spectra of solutions in CDCl 3 of dimethyl(phenylthio)arsine and tris(phenylthio)arsine where investigated. All resolved experimental resonances were assigned and the coupling constants determined by means of an iterative procedure employing the LAO-COON3 program. The spectra were simulated using Gaussian—Lorentzian lineshapes. The protons in the 2,6- and 3,5-positions were found to be equivalent. The resonances appear in the sequence δ 2,6 ⪢ δ 3,5 ⪢ δ 4 and δ 2,6 ⪢ δ 4 ⪢ δ 3,5 for dimethyl(phenylthio)arsine and tris(phenylthio)arsine, respectively. Whereas the protons in the 2,6-positions resonate at the same frequency (7.44 ppm) in the two compounds, the shifts for the 3,5- and 4-protons of tris(phenylthio)arsine are downfield from those of dimethyl(phenylthio)arsine. These differences are explained in terms of the hypothesis that the conformation of the rigid tris(phenylthio)arsine molecule does not allow overlap between the sulfur atomholding the lone electron pair and the π-ring orbital to the same extent as in dimethyl(phenylthio)arsine; in which the phenyl group can oscillate about an equilibrium position characterized by maximal p s -π-ring overlap.