The chemoselectivities of the reactions of iodine(1) acetate and iodine(1) acetate-thallium(1) acetate with cis-cyclooct-5-ene-1,2-diol have been investigated. The addition of iodine(1) acetate to cyclo-octa-1,5-diene, for a number of reagent systems, results in products arising from both 1,2-addition and transannular pathways.