This paper describes a dual beam stopped flow apparatus for quantitatively observing rapid (up to 250 sec−1) chemical reactions in turbid suspensions. The unit employs two commercial 150 W xenon arc light sources modulated to produce square light pulses at 5000 cps; two wavelengths of light are selected by monochromators, and the light is delivered to the reaction cuvette of a conventional stopped flow apparatus via a fiber optic light guide ``Y.'' Current and light intensity feedback are utilized to control lamp intensity; simple transistor choppers demodulate the measuring photomultiplier output, and a second photomultiplier is utilized in a differencing scheme to reduce effects of residual lamp instability. The instrument has a limit of detectability of 1×10−3optical density units; crosstalk of signal from one channel into the other channel is 6% of the change being measured. An example of instrument performance is presented; as now constructed, the instrument provides time resolution at least 5 times higher and reagent economy more than 20 times higher than other available dual wavelength spectrophotometers.