Blending organic molecules to tune their energy levels is currently investigated as an approach to engineer the bulk and interfacial optoelectronic properties of organic semiconductors. It has been proven that the ionization energy (IE) and electron affinity (EA) can be equally shifted in the same direction by electrostatic effects controlled by blending similar halogenated derivatives with different energetics. Here, we show that the energy gap of organic semiconductors can be tuned by blending as well. We use oligothiophenes with different numbers of thiophene rings as example and investigate their structure and electronic properties. Photoelectron spectroscopy and inverse photoelectron spectroscopy show tunability of the single-particle gap, with the optical gaps showing similar, but smaller effects. Theoretical analysis shows that this tuning is mainly caused by a change in the dielectric constant with blend ratio. Further studies will explore the practical impact of this energy-level engineering strategy in optoelectronic devices.