Modifications to a commercial electron paramagnetic resonance (e.p.r.) spectrometer are described which allow data to be obtained from 64 e.p.r, spectra (0.1–100 gauss per scan) with scan times of 0.010–0.900 s and with 0.020–900 s delay times between scans. Reagents are delivered into an e.p.r, cell from a stopped-flow mixer that triggers the generation of a microprocessor-controlled waveform to drive a rapid-scan unit. This digitally synthesized waveform is designed to correct inherent imperfections in the Helmholtz-type sweep coil circuit in the rapid-scan unit. The spectra are digitized (250 points per scan) and can be processed to determine rates of reaction. Performance of the system is demonstrated by the determination of the kinetics of rearrangement of a bis(di- peptide)nickelate(III) complex.