A dual beam flash photolysis system for studying the kinetics of transient species is described. The system has high sensitivity and is capable of detecting species absorbing 0.1% of the analyzing light with a noise level equivalent to 0.02% of the analyzing light. This high sensitivity has been achieved by producing an intense (6.2×1018photons in the uv) flash of short duration (0.9 &mgr;sec to decay to half height and 3 &mgr;sec to decay to 1%). We have also used an analyzing lamp which is intensified to 30 times its normal brightness and a long path (27 cm) through the solution. The photomultipliers have been adjusted to be linear up to 10 mA so that the potentialities of the intensified analyzing beam may be exploited. Spurious transients produced in the water have been reduced by careful distillation of the water followed by further purification through the photolysis reaction of the flash itself. The configuration of the apparatus and components is described in detail.