An electronic system is described that subtracts the analyzing light signal level from the analyzing light signal modulated by a transient absorption signal at the output of a photodetector in a transient absorption spectrophotometer. The system enables sensitive measurement of the transient with simultaneous measurement of the analyzing light level immediately before the transient. The shortest rise time is 50 ns. The time window for the measurement can be selected to a maximum of 20 ms with a droop of 20 &mgr;V. Input signals of 10 V of either polarity are permitted. The minimum detectable absorption signal is determined by the noise of the photomultiplier used and can be about 0.2% for an experiment without signal averaging.