A series of thiazolothiazole–phenylene co-oligomers was synthesized and their properties, particularly as semiconductors for OFETs, were investigated. The naphthyl and biphenyl- substituted derivatives showed p-type semiconducting behavior with hole mobilities ranging from 10⁻² to 10⁻¹ cm² V⁻¹ s⁻¹. The hole mobility and on/off ratio were enhanced to 0.12 cm² V⁻¹ s⁻¹ and 10⁶ on the HMDS treated substrate. The introduction of phenyl groups as end substituents was found to be favorable for charge transport and air-stability.