Transition processes have been studied in gas sensor models based on catalytically active electrodes (cae) and hydrogenated solid proton electrolyte in reaching current-free potentials and the current of chlorine and ozone reduction. A reversible chlorine adsorption and irreversible ozone adsorption and decomposition have been shown to proceed at a cae of Pt, RuO2 and their compositions with TiO2. It has been established that in the conditions of real humidity of the air caes possess the properties of porous electrodes with poorly pronounced ionic conductivity. The rate of cathodic reactions at the cae is limited by the external diffusion regime of mass transfer in gas phase and by the internal diffusion-ohmic regime of mass transfer of chlorine, ozone and products of their reduction. It has been shown that the time of transition processes in the systems under consideration is determined from the time of reaching an adsorption equilibrium in those sections of the cae where the reduction of chlorine and ozone does not occur.