Abstract When 5-substituted 5,10-dihydrophenarsazines are converted to their arsonium salts with methyl iodide the NH proton in the 1H NMR spectrum undergoes a downfield shift (4.1–4.2 ppm) too large to be accounted for by inductive effects alone. A similar shift is observed for an ( o- and a p-aminophenyl)arsine but not in an o-phenylenediamine. To account for this, d π- p π overlap similar to that found in phosphonitrilic compounds is suggested. The ultraviolet spectra of both the arsines and the arsonium salts are similar and show a long wavelength band (> 300 nm) which is attributed to the presence of d π- p π bonding in both classes of compound. Again, nitrogen analogues show no comparable effect.