The atomic composition and the electronic properties of ultrathin films of the four-conjugated rings thiophene/phenylene co-oligomers end-terminated by the −CH₃ and by the -CF₃ radicals vacuum deposited onto the oxidized silicon surface were studied using the X-ray photoelectron spectroscopy (XPS) and total current spectroscopy (TCS) techniques. The relative concentrations of the C, S and F atoms measured from the film deposits were evaluated and were found to correspond to the chemical formulae of the CH₃-phenylene-thiophene-thiophene-phenylene−CH₃ (CH₃-PTTP−CH₃) and CF₃-phenylene-thiophene-thiophene-phenylene-CF₃ (CF₃-PTTP-CF₃) molecules studied. The TCS measurements during the increase of the organic layer thickness up to 6 nm on a (SiO₂)n-Si substrate were used to determine the conduction band peak structure related to the density of the unoccupied electronic states (DOUS) in the range from 5 eV to 22 eV above E_F. DOUS of the films investigated was also obtained theoretically by means of ab initio calculations. According to the calculations, the energy position of the lowest unoccupied molecular orbital (LUMO) peak was found at 4.4 eV in the case of the CH₃-PTTP−CH₃ film and at 3.9 eV in the case of the CF₃-PTTP-CF₃ film. The effect of fluorine substitution on the DOUS was determined as approximately 2 eV peak shift towards low electron energies in the case of the CF₃-PTTP-CF₃ film compared to the case of the CH₃-PTTP−CH₃ film in the electron energy range from 10 eV to 22 eV above E_F and as a more complex peak restructuring in the energy range from 5 eV to 10 eV above E_F. The DOUS peaks of the two films studied were assigned to molecular orbitals with π* character in the 5–9.5 eV energy range and molecular orbitals with σ* character in the 9.5–22 eV energy range above E_F.