Organic transistors based on blends of 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) and poly(triarylamine) [PTAA] are strong candidates for application in future large-volume organic electronics because of their record breaking carrier mobility and environmental stability. Here we report on the dependence of hole transport on the composition of diF-TES ADT:PTAA blends. Maximum mobility is obtained only for blends containing ⩾39 wt.% diF-TES ADT. At compositions below this threshold the hole mobility is drastically reduced and is found to be equal to that of the neat polymer matrix, i.e. PTAA (∼10−3 cm2/V s). Scanning atomic force and polarised light microscopy, combined with differential scanning calorimetry, show that this threshold corresponds to the appearance of crystalline, acene-rich regions in such blend films, and the formation of high mobility conduction pathways between the source and drain electrodes. The dependence of hole mobility on diF-TES ADT concentration can be modelled using a simple percolation model. The present results provide important insights into the microstructure evolution in this high performance semiconducting blend and could prove valuable for the development of the next generation organic transistors.