AbstractThe aggregation behavior of pseudoisocyanine chloride (= PIC) (1,1′‐diethyl‐2,2′‐cyanine chloride) in dilute aqueous solutions was observed up to 375 MPa by absorption spectroscopy. From the pressure dependence of the dimerization constant the change in volume associated with the dimerization, ΔVD= −12.6 cm3·mole−1, could be obtained. By means of this value of ΔVDthe interplanar spacing of the quinoline rings in the dimer was estimated to 3.0·10−8cm. At higher concentrations the pressure dependence of the pressure‐induced absorption‐band (Scheibeband) yields the volume contraction (ΔVS) caused by the aggregation. Evaluation of ΔVS/ ΔVDallows a rough determination of the mean aggregation numbernof monomer units per Scheibe‐aggregate.nincreases with increasing concentration of PIC. The v