The stabilization of the light sources and the photocell circuits of a quartz spectrophotometer and the design of suitable stopped flow apparatuses are described. The latter have been designed for 1.6‐ and 10‐mm optical paths and have been shown to give rapid mixing (<0.1 sec). Errors of optical density measurement are diminished by rectangular observation tubes, and errors of extinction coefficients are minimized by a narrow spectral interval (less than 2 m&mgr; from 230–600 m&mgr;). The error of optical density caused by the photoelectric circuits is about 1×10−4for a response time (10–90 percent) of 1 sec. The application of this apparatus to the measurement of rapid chemical reactions in the visible and ultraviolet regions is described.