(6) arose on exposure of the primary adduct 4 to dilute acetic acid; a direct route to 6 was provided by oxidation of 1-iminoallantoin (2) with iodine. Both dehydro-allantoin (8) and its 4-imino analogue 6 form covalent adducts (e.g. with MeOH) at the 5-position. A brief rationale which focusses upon the stepwise nature and regiochemical course of the reaction is presented.