It is possible to perform a potentiomeric measurement in the gas phase in such a way that the formation of charge transfer complex between the gaseous species and semiconducting electrode results in chemical modulation of the Fermi level. In such a case the functional relationship is a generalized form of the Nernst equation in which the slope of the dependence of measured potential on partial pressure of gas assumes fractional values. It is shown that an entire new class of solid state gas sensors can be based on this principle.