The electronic structures of three 2,5-disubstituted thiophenes (2-bromo-5-chlorothiophene,2-bromo-5-methylthiophene and 2-bromo-5-nitrothiophene) have been studied by ultraviolet photoelectron spectroscopy (UPS).Combined the empirical arguments and theoretical method,the observed bands were interpreted.The analysis of electronic effects of the donor/acceptor substituted groups is essential for the reliable assignment of the observed photoelectron spectra.The highest occupied molecular orbital (HOMO) for each compounds is mainly the ionization of π3 orbitals.The analysis of the natural bond orbital (NBO) second-order interaction energies has also been employed to understand the nature of the intermoleeular interactions between the substitutents and the parent ring.The investigation of π and n orbitals ionization potentials enabled us to describe the correlation between substituent effects and relative reactivities of the substituted thiophenes.