Thin chalcogenide (As-S, As-Se, Ge-Se, etc.) films find applications in many branches of modern optics: for design of optical systems operating in VIS and IR, as recording material for holographic storage, or as inorganic photoresists, etc. Very thin films are included in multi-layered CD-R and DVD structures for improvements of their performance and increase of storage capacity. That is why developments of different methods for determination of optical constants (n - refractive index, k - extinction coefficient and d - physical thickness) best adapted for concrete optical problems are still needed. Recently we have presented a method for (n, k, d) evaluation of very thin metal or semiconductor films from spectrophotometric data. Here we present investigation of the optical constants of vacuum deposited As-Se thin
layers with d between 15 and 30 nm. The dispersion of the complex refractive index is studied in the spectral range of 400 - 1000 nm. The obtained results are interpreted within the frame of single oscillator Wemple-DiDomenico model. Comparison is made with data on thicker evaporated layers. We demonstrate the importance of the analysis of the uncertainties in (n, k, d) determination for the adequate choice of the film physical thickness.