Triphenylarsine hydroxybromide and hydroxychloride are weak electrolytes in methyl cyanide, the ions produced being Ph3AsOH+ and Hal–. Conductimetric titrations of these compounds with bromine and iodine chloride respectively indicate that the halogen adducts Ph3As(OH)Br,Br2 and Ph3As(OH)Cl,ICI are strong electrolytes of the form Ph3AsOH+Hal3– in methyl cyanide. The adducts have been obtained as solids. The perchlorate and tetrahalogenomercurate(ii) derivatives of the [Ph3AsOH+] ion have been isolated. A bromine adduct of the unusual composition Ph3As(OH)Br,O·5Br2 was obtained and for this the structure [Ph3AsOH+]2Br3–Br– is suggested. Infrared spectra are recorded.