Abstract: The invention relates to an apparatus for determining concentrations (x1,x2) of components of a gas mixture (2) flowing through a pipeline (1), wherein the gas mixture (2) is essentially composed of two or three components, wherein an ultrasonic, flow measuring device (31) is provided, which determines velocity (Cg) of sound of the gas mixture (2) flowing in a pipeline (1), wherein a temperature measuring unit (12) is provided, which determines temperature (T) of the gas mixture (2) flowing in the pipeline (1), and wherein an evaluation unit (21) is provided, which, based on velocity of sound (Cg) ascertained via ultrasonic measurement and based on velocity of sound (Ct) resulting from evaluation of the real gas equation, determines the concentrations (x1,x2) of the components of the gas mixture (2).