Cls core level photoelectron studies have been performed in the gas phase for some conjugated organic molecules, namely phenylacetylene, diethynylbiphenyl, diphenylbutadiyne and para-nitrophenylacetylene. The investigated systems consist of alkyne molecules with aromatic substituents and have been used as precursors for the synthesis of organic K-conjugated polymers. The experimental results were interpreted with the help of theoretical calculations: the ionization potentials have been predicted for all the chemically different carbon atoms of the organic molecules by means of Delta SCF quantum chemical calculations and compared with the measured binding energies of the various features present in the experimental spectra. A good agreement between experiment and theory was found.