Co-crystals of organic semiconductors can provide model systems for the study of fundamental optoelectronic properties and also new functionality. 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) and its fluorinated analogue 1,2,3,4,8,9,10,11-octafluoro-bis(triisopropylsilylethynyl)pentacene (F8TIPS-Pn) form crystalline substitutional alloys during bulk crystal growth from solution. Alloys can also be formed by blade casting thin films from solution. The compounds appear to have sufficient geometric similarity to be miscible in all proportions without inducing long-range disorder in the solid state. The offsets of the electronic levels of TIPS-Pn and F8TIPS-Pn in pure form are similar to those found in bulk heterojunction solar cells. UV/ vis spectroscopy and density functional theory demonstrate the charge transfer absorption in the alloy crystals corresponds to an excitation across molecular pairs.