Details are reported of the construction, calibration and testing of a fast-scanning spectrometer consisting of adouble-diodearrayopticalsimultaneousmultichannelanalyzer (DDA-OSMA) coupled to stopped-flow instrument. This set-up enables the recording of high quality absorbance spectra as well as absorbance-time traces as a function of wavelength, on a millisecond time scale. The electron-transfer reaction between Fe(CN)63-and L-ascorbic acid is used to demonstrate the performance of the system.