The invention relates to a device and a method for monitoring, controlling and/or regulating a gas composition. Said device comprises at least one training and/or rehabilitation unit a sensor unit comprising a heatable electrochemical solid electrolyte sensor for oxygen concentration determination and another heatable electrochemical solid electrolyte sensor for carbon dioxide concentration determination a control unit for the sensors a microcontroller in the control unit for controlling, according to the respiratory flow volume, the heating power of heating elements of the sensors in order to maintain constant sensor temperatures and a system for enriching and/or depleting air constituents for producing the gas composition, according to determinable nominal values and/or according to the respiratory gas composition determined by the sensor unit, and the determined respiratory flow volume.
