Sensor for determining an air ratio of a combustible gas/air mixture, the sensor comprising: a housing which limits a measuring space, a gas permeable separation means, wherein said gas permeable separation means forms a diffusion passage of the housing, wherein the diffusion passage is arranged to couple a combustible gas/air mixture flow with the measuring space, an electrically operated stimulating means for an energy supply into the measuring space, in order to bring about a chemical reaction of a combustible gas/air mixture in the measuring space, wherein the stimulating means is adapted to effect the energy supply at a reaction position situated in the measuring space, the reaction position being spaced from the housing and the separation means, wherein the stimulating means is formed by constructively and galvanically separated excitation electrodes (10, 11) which protrude at least partially into the measuring space, wherein the reaction position lies between the electrodes (10, 11), a voltage supply coupled with the stimulating means, at least one optical sensing device having a the sensing area, said sensing area is directed in the measuring space onto the reaction position, wherein the optical sensing device sensing the intensity of radiation from the reaction position and generates a signal assigned to the sensed intensity, from which the air ratio can be calculated, characterized in that the voltage supply is coupled with the excitation electrodes (10, 11) so that it is arranged to provide a microwave field in the reaction position such that, in the reaction position, an exothermic chemical reaction is induced by the microwave field in the combustible gas/air mixture. a housing which limits a measuring space, a gas permeable separation means, wherein said gas permeable separation means forms a diffusion passage of the housing, wherein the diffusion passage is arranged to couple a combustible gas/air mixture flow with the measuring space, wherein said gas permeable separation means forms a diffusion passage of the housing, wherein the diffusion passage is arranged to couple a combustible gas/air mixture flow with the measuring space, an electrically operated stimulating means for an energy supply into the measuring space, in order to bring about a chemical reaction of a combustible gas/air mixture in the measuring space, wherein the stimulating means is adapted to effect the energy supply at a reaction position situated in the measuring space, the reaction position being spaced from the housing and the separation means, wherein the stimulating means is formed by constructively and galvanically separated excitation electrodes (10, 11) which protrude at least partially into the measuring space, wherein the reaction position lies between the electrodes (10, 11), a voltage supply coupled with the stimulating means, at least one optical sensing device having a the sensing area, said sensing area is directed in the measuring space onto the reaction position, wherein the optical sensing device sensing the intensity of radiation from the reaction position and generates a signal assigned to the sensed intensity, from which the air ratio can be calculated, characterized in that the voltage supply is coupled with the excitation electrodes (10, 11) so that it is arranged to provide a microwave field in the reaction position such that, in the reaction position, an exothermic chemical reaction is induced by the microwave field in the combustible gas/air mixture. wherein the stimulating means is adapted to effect the energy supply at a reaction position situated in the measuring space, the reaction position being spaced from the housing and the separation means, wherein the stimulating means is formed by constructively and galvanically separated excitation electrodes (10, 11) which protrude at least partially into the measuring space, wherein the reaction position lies between the electrodes (10, 11), a voltage supply coupled with the stimulating means, at least one optical sensing device having a the sensing area, said sensing area is directed in the measuring space onto the reaction position, wherein the optical sensing device sensing the intensity of radiation from the reaction position and generates a signal assigned to the sensed intensity, from which the air ratio can be calculated, characterized in that the voltage supply is coupled with the excitation electrodes (10, 11) so that it is arranged to provide a microwave field in the reaction position such that, in the reaction position, an exothermic chemical reaction is induced by the microwave field in the combustible gas/air mixture.