A method for regulating an air-fuel ratio of an internal combustion engine, comprising the steps: Detection of an actual probe signal (U Ist) of an exhaust gas probe dependent on the exhaust gas composition of the exhaust gas of an internal combustion engine, determination of the exhaust gas composition (λ Ist) as a function the actual probe signal (U Ist) by means of a characteristic curve or a calculation rule, comparison of the determined exhaust gas composition (λ Ist) with a target value (A Soll) or threshold value, manipulation (116, 118, 120) of the air-fuel ratio supplied to the internal combustion engine, if the determined exhaust gas composition (λ Ist) attains or exceeds the target value (λ Soll) or threshold value, wherein a safety margin (ΔS), which takes into account an impact of at least one disturbance variable on the actual probe signal (U Ist), is defined, and the safety margin (ΔS) is applied to the characteristic curve, the calculation rule, the actual probe signal (U Ist), the target value (λ Soll) or threshold value, characterized in that an evaluation of the current accuracy of the at least one disturbance variable and/or a current impact of the at least one disturbance variable on the probe signal (U Ist) is performed and the safety margin (ΔS) owing to the at least one disturbance variable as a function of the evaluation is variably defined, wherein for the evaluation of the current accuracy of the at least one disturbance variable a spreading width of detected values of said disturbance variable in a past period of time is determined and the safety margin (ΔS) is defined as a function of the spreading width, or for the evaluation of the current accuracy of the at least one disturbance variable a duration is determined, which has elapsed since a past calibration of a detection system of said disturbance variable, and the safety margin (ΔS) is defined as a function of the duration, or for the evaluation of the current impact of the at least one disturbance variable on the probe signal an absolute height of the currently detected disturbance variable is determined and the safety margin (ΔS) is defined as a function of the absolute height. Detection of an actual probe signal (U Ist) of an exhaust gas probe dependent on the exhaust gas composition of the exhaust gas of an internal combustion engine, determination of the exhaust gas composition (λ Ist) as a function the actual probe signal (U Ist) by means of a characteristic curve or a calculation rule, comparison of the determined exhaust gas composition (λ Ist) with a target value (A Soll) or threshold value, manipulation (116, 118, 120) of the air-fuel ratio supplied to the internal combustion engine, if the determined exhaust gas composition (λ Ist) attains or exceeds the target value (λ Soll) or threshold value, wherein a safety margin (ΔS), which takes into account an impact of at least one disturbance variable on the actual probe signal (U Ist), is defined, and the safety margin (ΔS) is applied to the characteristic curve, the calculation rule, the actual probe signal (U Ist), the target value (λ Soll) or threshold value, characterized in that an evaluation of the current accuracy of the at least one disturbance variable and/or a current impact of the at least one disturbance variable on the probe signal (U Ist) is performed and the safety margin (ΔS) owing to the at least one disturbance variable as a function of the evaluation is variably defined, wherein for the evaluation of the current accuracy of the at least one disturbance variable a spreading width of detected values of said disturbance variable in a past period of time is determined and the safety margin (ΔS) is defined as a function of the spreading width, or for the evaluation of the current accuracy of the at least one disturbance variable a duration is determined, which has elapsed since a past calibration of a detection system of said disturbance variable, and the safety margin (ΔS) is defined as a function of the duration, or for the evaluation of the current impact of the at least one disturbance variable on the probe signal an absolute height of the currently detected disturbance variable is determined and the safety margin (ΔS) is defined as a function of the absolute height.