The absorption and fluorescence spectra of 1,1'-diethyl-2,2'-cyanine (pseudoisocyanine, PIC) aggregates have been studied between 8 and 293 K in water/glycerol glass containing 2-4 M of alkali halogenides. In this system the J-aggregates have a single sharp band and there is practically no contamination with the monomeric dye, dimers, or II-aggregates. This allowed us to better resolve the high-energy portion of the spectrum and to assign the middle 535 nm band to the upper exciton transition. The excitonic splitting at 8 K is the same for both the blue and the red forms of aggregates (1270 +/- 10 cm(-1)). The average energy of exciton components (18195 +/- 15 cm(-1) for the blue form) was found to be very close to the 0-0 energy of the first strong site of PIC monomer (18223 cm(-1)) embedded in a 9-aza-PIC iodide matrix, which is transparent above 500 nm [Marchetti, A. P.; Scozzafava, M. Chem. Phys. Lett. 1976, 41, 87]. The 0-0 frequency of the nonsolvated PIC monomer cation (nu(0)(0) = 19716 +/- 40 cm(-1) or 507.2 +/- 1 nm) was obtained from the solvent shift measurements at room temperature. The absorption bandwidths and shifts of both the PIC cation in poly(methyl methacrylate) matrix and the aggregates were recorded in;the temperature range between 8 and 300 K. The thermal shift of band maxima was analysed in terms of the change in dispersive shift and excitonic splitting as a result of the expansion of the matrix and a pure thermal or phonon-induced contribution. The thermal shift and broadening behavior of molecular and excitonic transitions reveals large differences in the mechanism and strength of the coupling-to low-frequency vibrations.