The SX20 is used to study transient and pre-steady-state kinetics of fast, liquid-phase chemical and biochemical reactions initiated by the rapid mixing and stopping (stopped-flow) of the reactants. A spectroscopic probe (absorbance or fluorescence) is employed to follow the course of the reaction by recording changes in the amplitude of the spectroscopic signal as a function of time. A typical upper limit to the reaction rates that can be measured with stopped-flow is ~2000s-1 in standard configuration; with smaller volume cells, rates in excess of 3000s-1 can be measured. The range of applications for stopped-flow spectroscopy is huge and many thousands of examples of its use can be found in the literature (see SX-series list of peer-reviewed publications). The study of enzyme catalysis, protein refolding, signal transduction, ligand or drug binding to proteins or DNA and kinetics of coordination chemistry are numbered among the many applications of stopped-flow spectroscopy. The SX20 is the undisputed market leader in stopped-flow. Applied Photophysics has supplied more than half of all the stopped-flow instruments in use today and to date these instruments have been used to generate more than 2000 scientific publications.