A series of 5′-aryl-substituted 2, 5-bis(3′-hexylthiophen- 2′-yl)thiazolo[5, 4-d]thiazole derivatives was synthesized and these expanded semiconductors were investigated as active materials for solution-processable organic field-effect transistors. Field-effect mobilities of up to 10 cm V s were obtained, representing the first reasonable FET behavior for highly soluble thiazolo[5, 4-d]thiazole-based small organic compounds suitable for printable electronics. Thermal and electrooptical material properties were studied by thermogravimetric analysis, differential scanning calorimetry, cyclic voltammetry, and UV/Vis spectroscopy. Trends in thermal and optical data were supported by (time-dependent) density functional theory calculations. Additional X-ray diffraction, atomic force microscopy, and scanning electron microscopy studies provided insight in the relationship between the molecular structures, film morphologies, and FET performances. The fibrillar microcrystalline structure observed for the best-performing thienyl-substituted material was linked to the high mobility.