Alkalimetric, spectrophotometric, NMR, and resonance Raman titrations are reported for the sulfonamide Neoprontosil in aqueous solution. An assignment of the magnetic resonance peaks for each of the Neoprontosil protons has been made. Neoprontosil is shown to have two "coupled" iity of the microscopic pKs for these two groups precludes spectroscopic characterization of the separate -SO2NH2, -O- or -SO2NH-, -OH species. For this reason, no conclusion can be drawn on the ionization state of the drug when bound to carbonic anhydrase. The resonance Raman spectrum of Neoprontosil bound to human carbonic anhydrase B at pH 9.5 shows a shift in the intense -N=N- stretching mode from 1414 (free) to 1407 cm- (bound), suggesting that a slight conformational change about the -N=N- single bond linkages occurs upon binding.