A continuous vertical scanning photosensitive device is described which is capable of monitoring the radiation intensity of chemiluminescent reactions in cylindrical flow reactors. The output from the photomultiplier is fed to a conventional millivolt recorder to obtain a plot of intensity as a function of time. This enables calculations to be made of concentrations and rate constants of reaction for the species taking part in the chemiluminescent reaction, if the reactions occurring are well understood. The system increases the accuracy of measuring rates of reaction, greatly shortens the time required to carry out an experiment, and is inexpensive to construct.