Details on the construction of a spectrophotometer for measuring fast absorption changes occurring during photo‐synthesis are described. Repetitive, intense light flashes with 20‐μsec duration were used to initiate the reaction. The pulse generator is capable of producing two flashes, either simultaneously or with variable time separation. The monochromatic monitoring light was split into the sample beam and the reference balancing beam. The photo‐multiplier outputs were fed to a differential circuit so that net absorption changes could be measured. Besides a wideband oscilloscope, a computer of average transients was also used to extract small signals from a high noise background. Thus, transient optical density changes as small as 0.0001 could readily be measured. The performance of the apparatus, especially in conjunction with the CAT computer, was illustrated with the absorption change at 515 mμ in Chlorella and spinach chloroplasts.