Transient method and temperature-programmed desorption (TPD) measurements based on the response of the AC electrical conductance (G) have been used to characterise a H-2-Pt/Al2O3 system. It was shown that if the surface was insufficiently cleaned and contained traces of oxygen, the transient response H-2 (inc., 0) - G was of the overshoot type. Its characteristics are related with the involvement of some competitive processes controlled by the highly exothermic parasite reaction between hydrogen and oxygen. The "ignition" reaction O-2-H-2 produces the sharp increase of the amount of the spillover hydrogen, which decreases by back spillover and desorption as much as the "extinction" is in progress. The G-TPD profiles, registered after H-2 (inc., 0) - G transient experiments and in conditions simulating the reaction sequence of the proposed mechanism, provide additional support for this statement.