A time-resolved stopped-flow x-ray absorption fine structure (XAFS) system was developed using synchrotron radiation as an x-ray source. The shortest sampling time for time-resolved measurements was on the order of 1 ms with the dead time of ∼5 ms, which was limited by the mixing procedure in the stopped-flow unit and by the drift velocity of the ions generated in the ionization chamber as a detector. The time-resolution ability and the quality of the obtained XAFS spectra were greatly improved in comparison with the laboratory stopped-flow XAFS apparatus previously constructed. The time-resolved XAFS measurements were performed for the reduction reaction of[Fe(CN)6]3−withL-ascorbic acid in an acidic aqueous solution. The rate constants obtained from the XAFS experiments were reasonable judging from the kinetic parameters previously reported. ©1997 American Institute of Physics.