A method to determine a mass proportion of a first component of a multi-component fluid is provided that includes separating the first component in a separation step at least in part from the multi-component fluid, determining at least two reference flow rates, selected from a flow rate of the multi-component fluid supplied to the separation step, a flow rate of a residual fluid resulting from the separation of the first component, and a flow rate of a separation fluid accumulating in the separation step, and determining the mass proportion of the first component from the selected reference flow rates, taking into account a non-separated residual content of the first component in the residual fluid. An extraneous content of an additional separated component of the multi-component fluid is additionally taken into account.