Reactions of the hydrido(carboxamido)triosmium cluster (μ-H)Os 3 (CO) 10 (μ-CNHCHMe 2 ) (1) with bromine and iodine have been studied. The reactions yield in the first stage dinuclear compounds (μ-X)Os 2 (CO) 6 (μ-OCNHCHMe 2 )X 2 (X=Br, 2a-cis; X=I, 2b-cis) with two halogen atoms in the cis form. Compound 2-cis is a kinetic product and in solution slowly transforms to a trans isomer (2-trans).