ESR fast-flow spectrum of 3-methyl indole radical cation produced in a system of 3-methyl indole, Ce4+, acetic acid, and H2SO4 having concentrations of 0.5 mM, 0.25 mM, 1.75 M, and 0.113 M, respectively. Equal volumes of aqueous solutions of 3-methyl indole/acetic acid and Ce4+/H2SO4 were mixed milliseconds before entering the ESR flat cell at a total flow rate of 50 mL/min. ESR spectra were recorded at 20 mW microwave power, 0.10 mT field modulation, 8.00 mT field sweep width, 163 ms time constant, 81 ms conversion time, and 79 scans of 1024 data points (Red). Also shown is the 3-methyl- indole radical cation simulation spectrum with coupling constants given in Table 1 (Green).