The growth of anhydrous uric acid (UA) and uric acid dihydrate (UAD) crystals from supersaturated aqueous solutions containing methylene blue, a cationic organic dye, has been investigated. Low concentrations of dye molecules were found to be included in both types of crystal matrixes during the growth process. Incorporation of dye into UA crystals occurs with high specificity, affecting primarily 10011 and 12011 growth sectors, while UAD crystals grown from solutions of similar dye concentration show inclusion but little specificity. The orientation of the UA-trapped species was determined from polarization data obtained from visible light microspectrometry. To achieve charge neutrality, a second anionic species must also be included with the methylene blue into UA and UAD crystal matrices. Under high pH conditions, crystallization of 1:1 stoichiometric mixtures of methylene blue and urate yields methylene blue hexahydrate (MBU-6H(2)O). The crystal structure of MBU-6H(2)O reveals continuous pi-pi stacks of planes of dye cations and urate anions mediated by water molecules. This structure provides an optimal geometry for methylene blue-urate pairs and additional support for the incorporation of these dimers in uric acid single-crystal matrices. The strikingly different inclusion patterns in UA and UAD demonstrate that subtle changes in the crystal surfaces and/or growth dynamics can greatly affect recognition events.