Method for measuring at least one compound in an exhaust gas stream (220, 420) resulting from internal combustion, said method including: - by means of a first sensor (204, 404), measuring a first relative proportion of oxygen in the exhaust gas stream (220, 420), the method being characterised in: - downstream said first sensor (204, 404), adding a defined flow of air to said exhaust gas stream (220, 420), - by means of a second sensor (209, 409), measuring a second relative proportion of oxygen in the combined stream of said exhaust gas stream (220, 420) and said added air, - determining a mass flow of oxygen in said exhaust gas stream (220, 420) resulting from said combustion in said internal combustion by means of said first and second relative proportions of oxygen, said defined flow of said air and a flow of said exhaust gas stream ( Q e), and - by means of a control unit, adjusting said defined flow of said air being added to the exhaust gas stream (220, 420) in dependence on said determined mass flow of oxygen in said exhaust gas stream (220, 420), the method further comprising: - determining the flow ( Q e) of said exhaust gas stream as: Q e = Q d C de − Q d C d C e − C de, where Q d is said defined flow of air, C d is the proportion of oxygen occurring in air, C e is said first relative proportion of oxygen, and C de is said second relative proportion of oxygen. - by means of a first sensor (204, 404), measuring a first relative proportion of oxygen in the exhaust gas stream (220, 420), the method being characterised in: - downstream said first sensor (204, 404), adding a defined flow of air to said exhaust gas stream (220, 420), - by means of a second sensor (209, 409), measuring a second relative proportion of oxygen in the combined stream of said exhaust gas stream (220, 420) and said added air, - determining a mass flow of oxygen in said exhaust gas stream (220, 420) resulting from said combustion in said internal combustion by means of said first and second relative proportions of oxygen, said defined flow of said air and a flow of said exhaust gas stream ( Q e), and - by means of a control unit, adjusting said defined flow of said air being added to the exhaust gas stream (220, 420) in dependence on said determined mass flow of oxygen in said exhaust gas stream (220, 420), the method further comprising: - determining the flow ( Q e) of said exhaust gas stream as: Q e = Q d C de − Q d C d C e − C de, where Q d is said defined flow of air, C d is the proportion of oxygen occurring in air, C e is said first relative proportion of oxygen, and C de is said second relative proportion of oxygen. - downstream said first sensor (204, 404), adding a defined flow of air to said exhaust gas stream (220, 420), - by means of a second sensor (209, 409), measuring a second relative proportion of oxygen in the combined stream of said exhaust gas stream (220, 420) and said added air, - determining a mass flow of oxygen in said exhaust gas stream (220, 420) resulting from said combustion in said internal combustion by means of said first and second relative proportions of oxygen, said defined flow of said air and a flow of said exhaust gas stream ( Q e), and - by means of a control unit, adjusting said defined flow of said air being added to the exhaust gas stream (220, 420) in dependence on said determined mass flow of oxygen in said exhaust gas stream (220, 420), the method further comprising: - determining the flow ( Q e) of said exhaust gas stream as: Q e = Q d C de − Q d C d C e − C de, where Q d is said defined flow of air, C d is the proportion of oxygen occurring in air, C e is said first relative proportion of oxygen, and C de is said second relative proportion of oxygen.