The efficient synthesis of decyl-end-capped thiophene-phenylene oligomers with different numbers and positions of 1,4-phenyl rings within a 2,5-oligothiophene conjugated chain, leading to electronic-grade pure semiconducting organic materials with improved oxidation stability, was elaborated. These are p-type organic semiconductors, showing high charge-carrier mobilities up to 0.4 cm 2 /Vs and on/off current ratios as large as 10 5 in organic thin-film transistors, as well as stage delays as low as 300 µs in the integrated circuits (ring oscillators). Long decyl end chains in these oligomers having 5-7 conjugated aromatic rings lead to limited solubility but improve the oligomers' crystallinity, making them promising semiconducting materials for organic electronic devices prepared by vapor evaporation. Investigation of the optical properties of the oligomers showed that they have a HOMO-LUMO energy gap approximately 0.2 eV higher than that of the corresponding oligothiophenes. Electrochemical measurements revealed a better oxidation stability of the thiophene-phenylene oligomers as compared to that of their oligothiophene analogues.