In the well-known classical determination of the index and thickness of thin transparent films formed on known substrates from the ellipsometric data, the film thickness is computed from its index value which is found first, independently of the former. However, these parameters of very thin films well under 100 Angstrom may not be accurately thus determined in the presence of experimental angular errors particularly that in the angle of incidence. In this paper, not only the film parameters but also the angle of incidence are found using a new method from the ellipsometric data measured on two film-absorbing substrate reflecting systems formed with the same transparent film but up to different thicknesses. The parameters obtained and their uncertainties estimated from the two methods are compared and consequently the limits of application of the methods are defined. The new method shows resistance to the propagation of angular errors to the parameters sought. Furthermore, a correlation detected between the error in the substrate real index and the uncertainty in the angle of incidence computed allows one to correct one of these by making use of the angle of incidence measured. These results are illustrated by applying the method presented to recently published experimental data.