The envelope method is a commonly used method for determination of some important optical constants, by using the envelopes of the transmittance T(lambda) and/or reflectance R(lambda) spectrum of the thin film deposited on transparent substrate. Two envelope methods were carried out in this paper: standard-method which assumes that substrate is absolutely transparent and modified-method which takes substrate absorption into account. The investigated sample is a uniform thin chalcogenide Cu-1[As-2(S0.5Se0.5)(3)](99) film, deposited onto two kinds of a weakly absorbing substrates that differ in thickness. It was shown that the degree of accuracy in determination of chosen optical parameters for both investigated samples is notably improved when the absorbance of the bare substrates is considered in the expressions for the envelopes. (C) 2009 Elsevier B.V. All rights reserved.