Some considerations of the spectroscopic performance of a soft x-ray Fabry-Pe´ rot e´ talon (XFPE) are presented. A whole study of the phase change on reflection of a soft x ray multilayer interferential mirror (MIM) has been necessary to evaluate the theoretical behavior of the order of interference of an XFPE. For grazing incidence angles, there is a linear dependence between the latter parameter and the grazing angle in the range of the diffraction Bragg peak. This property has enabled us to obtain local expressions for the finesse and the resolving power of an XFPE. Numerical simulations emphasize the interest of such a structure in x-ray spectroscopy.