Solid electrolyte potentiometry (SEP) is a valuable tool in the investigation of catalytic gas-solid reactions as it is an in situ technique applicable under working conditions. It is commonly used in combined kinetic and electrochemical measurements and relates the steady state value of the oxygen activity at the catalyst to the observed reaction rates. After the description of principles and experimental procedure the main results obtained so far are discussed, namely: (a) the chemisorption of oxygen and the stability of single phases in the open system far from equilibrium; (b) model considerations influenced by or initiated through the knowledge of the oxygen activity in steady state; and (c) additional insight in the catalyst behavior in the case of partial oxidations of organic compounds at oxidic catalysts.