AbstractOn increasing concentration in highly diluted aqueous solutions of pseudoisocyanine dyes an intensive absorption band of extremely small half‐width (Scheibe band) occurs which is attributed to the formation of dye aggregates (Scheibe or J‐aggregates). By polarizing microscopy it is shown that aqueous solutions of pseudoisocyanine diethylchloride at low temperatures and at concentrationsc>0.2 wt% become birefringent and exhibit a Schlieren texture characteristic for nematic liquid crystals. Above a certain clearing temperatureTcthe solution becomes optically isotropic during a discontinuous phase transition as shown by DSC. The Scheibe band vanishes at temperatures some degrees aboveTc. The phase behaviour is displayed in terms of aT‐c diagram. The aggregation numbers have been determined spectroscopically to about 40 to 50 only slightly increasing belowTc. Obviously, the Scheibe aggregates exhibit a long‐range orientational order even at high dilution typically for a nematic lyotropic liquid crystal. The phase diagram indicates that on increasing temperatures the orientational long‐range order vanishes first before the Scheibe aggregates will be thermically d