1. The proton magnetic resonance spectra (100 MHz) of dilute solutions of iodobenzene and iodobenzene dichloride in absolute dioxane were investigated. 2. In the iodobenzene molecule, only the ortho-protons are shielded more weakly than the protons of unsubstituted benzene, while in iodobenzene dichloride, the chemical shifts of the benzene protons are substantially greater than 7.27 p.p.m. 3. From iodobenzene to its dichloride, there is a sharp increase in the inductive constant of the substituent, while the change in the constant of the conjugation effect is negligible.