Losartan, the potassium salt of 2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4- yl)methyl]imidazol, is an efficient antihypertensive drug. The complexes of Losartan (C22H22ClN6O)withCu(II) and Co(II) are studied. The complexes Cu[(C22H22ClN6O)2.2H2O] 2H2O and Co[(C22H22ClN6O)2].2H2O are prepared in aqueous solution at room temperature. The complexes are investigated and characterized by infrared spectra, elemental analysis and thermogravimetric analysis. A general mechanism describing the formation and the thermal decomposition of these complexes are suggested.