The triplet-triplet (T-T) spectra of twelve symmetrical cyanine dyes have been recorded, using flash photolysis in the presence of triplet state sensitizers. Three sets of dyes have been studied, each set comprising four members which differ from one another by the length of the bridge joining two identical nuclei (quinoline or benzthiazole); the internuclear linking is made by one =CH-, or by a chain composed of three, five, or seven such groups. Excluding the shortest member of each set, the remaining nine dyes are found to follow a common trend: the wavenumber of the 0-0 vibrational peak of the most intenseT-Tabsorption band is linearly related to that of the 0-0 peak of the first singlet-singlet absorption band.