Available data on the structures, electronic substituent effects and I3C NMR spectra of aromatic polyvalent iodine compounds, particularly iodoso and iodoxy derivatives, are summarized and discussed. A series of aromatic tri-and penta-valent iodine compounds were synthesized, their 13C NMR spectra were measured and the substituent chemical shifts (SCS) for several polyvalent iodine functional groups were calculated. As the oxidation state of iodine increases (I + 111 + V), the strong shielding of the ipsocarbon atom by iodine in aryl iodides due to the 'heavyatom effect' (30 ppm) decreases substantially, and this decrease is apparently general for stable hypervalent iodine compounds. Possible explanations are discussed and correlations between chemical shifts, structures and/or electronic effects are proposed. KEY WORDS 13C NMR Polyvalent iodine compounds Substituent chemical shifts Hammett constants ous polymers.' '-' The only direct structural information available is a Mossbauer study which indicated the C-1-0 bond angle to be near 90 O . I 4