The branching topology of end groups attached to several series of oligothiophenes has a systematic effect on thermal and electrical properties of the oligomers. The series were synthesized in a modular approach and show a distinct drop of the melting point Tm on increasing bulkiness of the substituents. The same trend can be found for the dissociation temperatures Tdis of aggregates in solution. Similarly, monolayer OFET mobilities µFET are significantly decreasing with increasing bulkiness of the substituents. A simple geometric model is presented quantitatively correlating the transition temperatures and mobilities with the substituents’ structure based on a bulkiness parameter P, which allows predicting Tm, Tdis, and µFET of corresponding not yet synthesized oligomers with branched substituents. This model might be generally applicable for end-capped rod-like conjugated oligomers.