A system (100) to determine concentrations of components of a multiphase fluid. The system (100) comprising a first flowmeter (108) configured to receive a fluid flow and to generate one or more measurements of the fluid flow. The fluid flow including a first liquid and a second liquid. The first and second liquids are immiscible; a separation vessel (114) configured to receive the fluid flow, and to at least partially separate the first liquid and the second liquid. A second flowmeter (106) is configured to receive either the second liquid or a mixture of the first liquid and the second liquid, through a valve (112) and to generate one or more measurements of either the second liquid or the mixture of the first liquid and the second liquid. A proportion of the second liquid to the first liquid in the mixture is greater than a proportion of the second liquid to the first liquid in the fluid flow. A third flowmeter (110) is configured to receive the first liquid and to generate one or more measurements of the first liquid. A data processing apparatus (124) is operatively coupled to the first flowmeter (110), the second flowmeter (106), the third flowmeter (110). The data processing apparatus is configured to perform operations comprising: receiving the one or more measurements of the fluid flow from the first flowmeter; receiving the one or more measurements of the second liquid or the mixture of the first liquid and the second liquid from the second flowmeter; receiving the one or more measurements of the first liquid from the third flowmeter, and determining a concentration of the second liquid in the fluid flow.