AbstractHeteroaggregations of the pair of cyanine dyes pinacyanol and thiacarbocyanine induced by the polyanions poly(potassium styrenesulfonate) and poly(potassium vinyl sulfate), and of methylene blue and acridine orange induced by mercuric chloride are described. In each case the characteristic of heteroaggregation, namely the enhancement of the metachromatic band of the partner dye having λmaxat relatively shorter wavelength at the expense of the absorbance at the metachromatic band of the other dye, is reflected in the difference spectra. The dependence of the shape of the metachromatic band of pinacyanol on the nature of the polyanion inducing metachromasia is also reflected in the heteroaggregation of this dye and thiacarbocyanine induced by the two different polyanions. Mercuric chloride, though not a polyanion, also induces strong heteroaggregation of methylene blue and acridine orange