The chemical states of organic semiconductors were investigated by total-electron-yield soft X-ray absorption spectroscopy (TEY-XAS); and first-principles calculations. The organic semiconductors, pentacene (C22H14) and pentacenequinone (C22H12O2), were subjected to TEY-XAS and the experimental, spectra obtained were compared with the Is core-level excited spectra of C and 0 atoms, calculated by a first-principles planewave pseudopotential method. Excellent agreement between the measured and the calculated spectra were obtained for both materials. Using this methodology, we examined the chemical states of the aged pentacene, and confirmed that both C-OH and C=O chemical bonds are generated by exposure to air. This result implies that not only oxygen but also humidity causes pentacene oxidation.