Atomic ratio determinations in a series of pure inorganic compounds have been derived from XPS peak intensity measurements taken by area via the so-called 'first principles model' (FPM) and the 'elemental sensitivity factors' (ESF) methods. The two methods show similar degrees of accuracy, with a typical error of about f10% when a set of experimental ESF is developed.A lower accuriicy, as expected, has been obtained when the literature ESF have been applied, Some discrepancy between the two approaches are evident when the kinetic energy separations of the XPS elemental peaks involved in the atomic ratio calculations is large. This aspect is discussed in the light of the instrumental responses which are of primary concern for the transferability of ESF results from one instrument to another.