The interaction of single- and double-charged anions (HAn−, An2−) of sulfonephthaleins (phenolsulfonephthaleins and its alkyl-substituted derivatives, viz., o-cresolsulfonephthalein and thymolsulfonephthalein) with single-charged cations (Ct+) of polymethines (pinacyanol, quinaldine red) was studied. The interaction in an aqueous solution affords heteroassociates of stoichiometric composition (Ct+)·HAn− and (Ct+)2·An2−. The association constants were estimated from the spectrophotometric data. The enthalpies of formation of dye and heteroassociate ions were calculated and the most probable structure of the heteroassociates was deter- mined using the semiempirical methods.