The intermolecular interactions of the bis-azo dye Direct Blue 1 (Chicago Sky Blue 6B) have been studied as a function of concentration in aqueous solution and in cellophane using UV-visible absorption, NMR, and resonance Raman spectroscopy. UV-visible spectroscopy indicates that dimerization occurs in aqueous solution (K-dim approximate to 77 000 dm(3) mol(-1) at I = 0.01) and that it occurs more readily at higher ionic strength, where trimerization also occurs (K-dim approximate to 580 000 dm(3) mol(-1) and K-trim approximate to 2700 dm(3) mol(-1) at I = 0.1); the driving force is enthalpic rather than entropic (DeltaH(dim) approximate to -53 kJ mol(-1) and DeltaS(dim) approximate to -90 J K-1 mol(-1) at I = 0.01). Dimerization occurs much less readily in cellophane (K-dim approximate to 42 dm(3) mol(-1)) than in aqueous solution, indicating that strong dye-cellulose interactions compete effectively with dye-dye interactions. NMR spectroscopy indicates that Direct Blue 1 molecules interact by pi-stacking at the central biphenyl group, while resonance Raman spectroscopy indicates that the internal structure and bonding of the monomers is essentially retained on stacking. The UV-visible spectra are consistent with this interpretation, and the application of exciton theory indicates that stacking results in angles between adjacent molecules which are different in the dimer (theta approximate to 84degrees) and trimer (theta approximate to 58degrees); they are attributed to geometries which minimize the repulsion between charged naphthylsulfonate groups.