High air stability is the key issue for the organic thin film transistors (OTFTs). Recently developed high-mobility organic semiconductors, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT) and dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), are much more stable than pentacene. We measured the ultraviolet photoelectron spectra (UPS) of DPh-BTBT and DNTT and revealed that their HOMOs are situated at deeper energy levels than that of pentacene, which contributes to their high stability. The UPS of DNTT is well reproduced by the DFT calculation, while that of DPh-BTBT is not fully reproduced. This difference is discussed in terms of dihedral angle between the phenyl groups and the BTBT ring. (C) 2013 Elsevier B. V. All rights reserved.