Abstract: In a process of liquid mixing, more especially for use in mixing the solvents of an eluent for liquid chromatography, the liquids are supplied from separate reservoirs (1-4) to a common conduit (13) via fluid flow control valves (5-8). Predetermined pressure drops are maintained across each valve by arranging appropriate pressures in the reservoirs (1-4) and the conduit (13). The rate of flow of each liquid is determined by setting each valve (5-8) to present a predetermined flow resistance calculated from the formula ##EQU1## where L=the flow resistance of the valve .DELTA.H=the pressure drop across the valve S=the specific gravity of the liquid and I=the desired rate of flow of the liquid. The calculation and automatic actuation of the valves is effected by an electronic control (14) incorporating a microprocessor which may be programmed to vary the liquid mixture with time to provide a desired elution gradient.