We describe the novel concept of momentary equilibrium (ME), a special event during a pulse-response transient experiment in which the non-steady-state rates of adsorption and desorption of a probe molecule are instantaneously balanced. In the absence of other reactions, any system with reversible adsorption will always pass through ME during a pulse-response experiment with effusion. We also suggest a new method for measuring the concentration of adsorption sites on heterogeneous catalysts and the corresponding equilibrium constant by observing momentary equilibrium in thin-zone temporal analysis of products (TZTAP) pulse-response experiments with modulated pulse intensity. The suggested method employs reversible adsorption of probe molecules, contrary to traditional methods of counting adsorption sites which utilize irreversible reactions.