Different compositions of as-obtained As₄₀S_60-xSe_x thin films (x = 0 at.%, 20 at.%, 40 at.%, and 60 at.%) with fixed thicknesses were deposited by a thermal evaporation technique. Inheterogeneities of thin-film thickness is a problem that includes significant errors of optical calculations unless there is an optical model that prevents these errors, and the consequent gross errors, in the measurement of optical constants. If not taken into account, this may lead to rather large calculated values for the absorption coefficient or the incorrect presence of the absorption-band tail, as well as to significant errors in the calculated values of the refractive index and film thickness. The optical properties of As₄₀S_60-xSe_x thin films have been determined utilizing measurements of the optical transmission spectra. Owing to the shrinking of the transmission spectra in both the medium and strong absorption regions, we have resorted to applying the optical wedge model for the determination of the film thickness with high precision that equals approximately 725 nm. This paper therefore presents formulae for the transmittance spectrum of a thin dielectric film of selected thickness covering a thick, non-absorbing substrate as well as its upper and lower envelopes. The effect of the content variation on the interference fringes of the transmittance spectrum is analyzed in detail. The electrical properties of the As₄₀S_60-xSe_x thin films have been studied in terms of measuring the temperature-dependent AC conductivity. Both the dielectric constants and dielectric modulus were investigated and are discussed for applications in optoelectronic devices. The change in electrical properties of As₄₀S_60-xSe_x thin films has been interpreted in terms of changed morphological and structural properties. The ratios of the elements were analyzed by comparing them with the actual weight ratios of the bulk material using EDX technology, in addition to the assessment of the Amorphic structure and composition characteristics of the films examined by the x-ray and scanning electron microscopy.