The conversion of a Cary 60 spectropolarimeter to a fast circular dichroism instrument (CD) using CD detection for standard rapid reaction techniques and for recording normal CD spectra is described. Although the optical setup remains nearly the same, the electronics are modified to increase the frequency response. The use of a 18‐kHz piezo‐optical birefringence modulator allows measurements of relaxation times down to 5 ms. As an example of the kind of data obtainable with this instrument, measurements are presented on the acid denaturation of hemoglobin in the time range of 100 ms.