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-1can be measured