The absorption and fluorescence spectra of terthiophene (TT) as well as three substituted terthiophene derivatives, namely 3,3"-dimethoxy-2,2＇:5＇,2"-terthiophene (DMOTT), 3,3"-dimethyl-2,2＇:5＇,2"-terthiophene (DMTT), and 3＇,4＇-dihexyl-2,2＇:5＇,2"-terthiophene (DI-ITT), isolated in an alkane matrix and in their aggregated forms are reported. The thermochromic properties of substituted oligothiophenes are reported for the first time. The optical changes observed for the isolated molecules in the alkane matrix are interpreted in terms of conformational changes caused by the packing effect. After aggregation, TT exhibits a splitting in the excitation spectrum, which is provoked by an excitonic effect. On the other hand, it is observed that the changes in the optical properties of substituted terthiophenes are close to those found in the alkane matrix. This strongly suggests that the aggregation process does not produce any significant optical change, other than those provoked by conformational changes, in the excitation spectra of these molecules. It is suggested that the presence of the side chains weaken the intermolecular interactions, which are responsible for the excitonic effect. Finally, the fluorescence spectra of all the molecules investigated show a new red-shifted band, when aggregated. The new emission cannot be explained by conformational changes and has its origin in the aggregation process itself.