High-precision determination of the mass proportion of a component in a multi-component fluid

To determine a mass proportion of a first component (W) of a multi-component fluid (F), the following is provided: separating the first component (W) in a separation step at least in part from the multi-component fluid (F), determining at least two reference flow rates (M1,M2), selected from a flow rate (m1) of the multi-component fluid (F) supplied to the separation step, a flow rate (m2) of a residual fluid (T) resulting from the separation of the first component (W), and a flow rate (m3) of a separation fluid (K) accumulating in the separation step, and determining the mass proportion (xw) of the first component (W) from the selected reference flow rates (M1, M2), taking into account a non-separated residual content (mw2) of the first component (W) in the residual fluid (T). According to the invention, an extraneous content (mD3) of an additional separated component (D) of the multi-component fluid (F) is additionally taken into account.