Interactions of the cationic dye methylene blue with mercuric chloride have been studied conductometrically, analytically and spectrophotometrically. Methylene blue produces red colored precipitate with mercuric chloride; in presence of large excess of mercuric chloride a strong metachromasia is induced in the dye. Metachromasia induced by mercuric chloride is more hypsochromic as well as hypochromic than that induced by chromotopes like heparin. The complexes formed between methylene blue and mercuric chloride have variable compositions, the complex responsible for the red metachromatic color of the dye has the composition 2 dye: 1 HgCl2. A model has been proposed for the metachromatic complex consisting hexa-coordinated mercury, dye is coordinated to the mercury by donating the lone pair electrons of terminal nitrogen. The non-metachromatic dye capri blue also interacts with mercuric chloride but without any change in the visible spectrum. Potassium iodide also gives metachromatic reddish blue colored precipitate with methylene blue.