The Monsanto acetic acid process is one of the most effective ways to produce acetic acid industrially. This process has been studied experimentally but theoretical investigations are so far sparse. In the current work the active catalytic species [Rh(CO)(2)I-2](-) (1) and its isomerisation has been studied theoretically using the hybrid B3LYP exchange and correlation functional. Similar calculations has been performed for the iridium complex [Ir(CO)(2)I-2](-) (2) that also is catalytically active in the methanol carbonylation. Experimental work has confirmed the existence of the cis forms of the active catalytic species, but they do not rule out the possibility of the trans isomers. Our gas phase results show that cis-1 has 4.95 kcal/mol lower free energy than trans-1, and cis-2 has 10.39 kcal/mol lower free energy than trans-2, In the case of rhodium, trans-1 can take part to the catalytic cycle but in case of iridium this is not very likely. We have also investigated the possible mechanisms of the cis to trans conversions, The ligand association mechanism gave free energy barrier of 13.7 kcal/mol for the rhodium complex and 19.8 kcal/mol for iridium, Thus the conversion for the rhodium complex is feasible whereas for iridium it is unlikely, (C) 2001 Elsevier Science B.V. All rights reserved.