Chalcogenide glasses Ge-As-Se has a wide region of glass formation in respect with other ternary compounds, high glass transition temperatures Tg, and thermal stability [1]. The Ge-As-Se system is of interest due to high values of third order nonlinearities (c3), high refractive index (n=2.4¸2.65), high optical transmission at 1.55 mm, that makes it suitable materials for photonic devices [2]. Recently we have reported some optical properties of amorphous GexAsxSe2-x (x=0.05¸0.30) thin films prepared by thermal “flash” evaporation on the glass substrates held at Tsubstr=100 oC [3]. In the present work we present the optical measurements (transmission spectra T(l), absorption coefficient a(hn), optical band gap Eg), the calculated values of the refractive index n and its dependence on the composition and the degree of modification under light exposure, and the photodarkening effect. The transmission spectra was used for calculation of the absorption coefficient a, optical band gap Eg, and the values of the refractive index n. Fig. 1 shows a typical dispersion curve of the refractive index n(l) for the amorphous GexAsxSe2-x thin films. The plot (n2-1)-1 vs. (hn)2 (Fig.2) allow to determine the oscillator parameters by fitting a straight line to the experimental points.FigureFig.1. The dispersion curve of the refractive index n(l) for the amorphous GexAsxSe1-2x thin films. x: 1-0.07; 2-0.09; 3-0.11; 4-0.16; 5-0.30.Fig.2. The dependence of the refractive index (n2-1)-1 vs. (hn)2 for amorphous Ge0.07As0.07Se0.86 thin film.It was established that the optical band gap Eg decreases, while the refractive index n increases with the increasing of the concentration of Ge and As in the GexAsxSe2-x glassy system. The time dependence of the transmission T(t) during the light exposure (hn³Eg) is described by a strength exponential behaviour T(t)/T(0) = A0+Aexp[-(t-t0)/τ](1-b), where t is the exposure time, τ is the apparent time constant, A characterizes the exponent amplitude, t0 and A0 are the initial coordinates, and b is the dispersion parameter (0<b<1). Using the computer fitting the parameters of the strength exponential were determined.