Two different effective-core-potential methodologies, augmented by extra polarization functions were used at the MP2 and CCSD(T) levels of theory to investigate a number of iodine containing isomers of the type XOOI, XIO2 and XOIO (X = Cl, Br, I). The procedures, successfully tested first on the well studied Cl2O2 system, produced results for XIO2 isomers analogous to the corresponding Cl and Br species. The relative energetics however, indicates competing stability depending on the method, between the peroxide XOOI and the Y-shaped XIO2 form for ClIO2 and BrIO2 and between IOOI and IOIO for the I2O2 family.