The optimization of an X-ray transmission-cell design for high-resolution X-ray reflectivity measurements of the kinetics and thermodynamics of reactions at mineral–solution interfaces is presented. The transmission cell is equipped with a liquid flow system consisting of a pair of automated syringe pumps whose relative flow rates control the composition of a solution injected into the cell with ∼1% precision. The reflectivity measurements from the muscovite-(001)–solution interface at photon energies of 15–16.5 keV show that the cell is useful for probing interfacial ion adsorption–desorption experiments at a time scale of several seconds or slower. The time resolution is achieved with a small-volume (∼0.22 ml) reaction chamber to facilitate fast solution exchange. Additional reductions in reaction chamber volume will improve both the data quality by reducing X-ray absorption through the solution and the time resolution by increasing the solution exchange rate in the cell.