Method for determining a carbon monoxide fraction in an exhaust gas stream of a regeneratively fired industrial furnace, in particular a glass furnace, for a method for controlling and/or regulating the regeneratively fired industrial furnace, characterised in that: - a lambda probe (52, 52') is introduced into the exhaust gas stream, and a temperature of the lambda probe is provided, and a first calculation rule is a probe rule which uses the cell voltage and the temperature of the lambda probe on the basis of the Nernst equation, and by means of the first calculation rule an oxygen fraction in the exhaust gas is determined on the basis of a Nernst equation, and - at least one cell voltage is provided by means of cell voltage values (UL, UR) as a function of time, on the basis of which - by means of a second calculation rule a carbon monoxide fraction in the exhaust gas is determined, and an exhaust gas temperature and a fuel composition are provided, and the second calculation rule for a carbon monoxide fraction in the exhaust gas is a combustion rule which uses the fuel composition on the basis of a water-gas equation, whereby from the cell voltage values - at least one equilibrium value is determined and - at least one non-equilibrium value which is increased compared with the equilibrium value is determined, whereby the equilibrium value is used for determination of a first carbon monoxide fraction in the exhaust gas which characterises an equilibrium fraction, and the increased non-equilibrium value is used for determination of a second carbon monoxide fraction which characterises a non-equilibrium fraction, and - from the cell voltage the non-equilibrium value which is increased compared with the equilibrium value is determined above a threshold range as a function of time from cell voltage peaks of the cell voltage values, and - for identification of the cell voltage peaks an adjustable large number of cell voltage values for an integral time window in a signal evaluation are stored in a moving manner, whereby a maximum or an envelope value of the stored cell voltage values is used for determination of the current second carbon monoxide fraction in the exhaust gas which characterises the non-equilibrium fraction. - a lambda probe (52, 52') is introduced into the exhaust gas stream, and a temperature of the lambda probe is provided, and a first calculation rule is a probe rule which uses the cell voltage and the temperature of the lambda probe on the basis of the Nernst equation, and by means of the first calculation rule an oxygen fraction in the exhaust gas is determined on the basis of a Nernst equation, and - at least one cell voltage is provided by means of cell voltage values (UL, UR) as a function of time, on the basis of which - by means of a second calculation rule a carbon monoxide fraction in the exhaust gas is determined, and an exhaust gas temperature and a fuel composition are provided, and the second calculation rule for a carbon monoxide fraction in the exhaust gas is a combustion rule which uses the fuel composition on the basis of a water-gas equation, whereby from the cell voltage values - at least one equilibrium value is determined and - at least one non-equilibrium value which is increased compared with the equilibrium value is determined, whereby the equilibrium value is used for determination of a first carbon monoxide fraction in the exhaust gas which characterises an equilibrium fraction, and the increased non-equilibrium value is used for determination of a second carbon monoxide fraction which characterises a non-equilibrium fraction, and - from the cell voltage the non-equilibrium value which is increased compared with the equilibrium value is determined above a threshold range as a function of time from cell voltage peaks of the cell voltage values, and - for identification of the cell voltage peaks an adjustable large number of cell voltage values for an integral time window in a signal evaluation are stored in a moving manner, whereby a maximum or an envelope value of the stored cell voltage values is used for determination of the current second carbon monoxide fraction in the exhaust gas which characterises the non-equilibrium fraction.