We report the construction of a system to measure the circular dichroism of transient phenomena. The technique employs rapid modulation of the polarization of an analyzing beam, and consequently significant information can be obtained in a single run. The system is sensitive to changes in polarization strength of 2×10−5occurring in 1 msec. The particular system described has been constructed to follow transient structure changes in hemoglobin following laser photolysis of CO hemoglobin. The technique is quite general, however, and design details are presented which would be useful in any attempt to apply modulation spectroscopy to transient measurements.