show that low-intensity emission bands are observed for 6MP and Aza following excitation at 334 nm in the pH range from 5.5 to 9.8. The primary emission band for 6MP at 495 nm is assigned to phosphorescence emission because it exhibits a large Stokes' shift relative to the lowest-energy absorption band and can be quenched by O2. Another emission band is observed at ca. 370 nm, which is assigned to fluorescence emission because it exhibits a relatively small Stokes's shift and is not quenched by O2. The excitation spectra reported in Figures S1 to S3 evidence that the emission is an intrinsic property of the 6MP prodrug and not due to impurities. Fluorescence and phosphorescence quantum yields as a function of pH were not determined because the emission signals are within the detection limit of the spectrometer.