Two different effective-core-potential methodologies, augmented by extra polarization functions were used to investigate a number of iodine triatomics of the type XOI, XIO and IXO (X=Cl, Br, I). The procedures were tested first on the well-studied ClOCl, BrOBr, ClOBr and IOI systems and they were shown to reproduce the full-electron calculation results with reasonable consistency. The results exhibit the same tendencies as the corresponding C1 and Br analogues and produce the greater stability for the isomer with the least electronegative atom in the middle. (C) 2002 Elsevier Science B.V. All rights reserved.