Protonation equilibria of the neurotransmitter serotonin and its precursor 5-hydroxytryptophan (5-HTP) are characterized at the macroscopic and microscopic levels. 1H-NMR-pH and UV-pH titrations were carried out to determine the macroconstants. Microconstants were obtained by appropriate combination of interactivity parameters and model compounds, allowing the calculation of all the twelve microconstants, the eight microspecies concentrations, and three site-interactivity parameters of 5-HTP, for which no microconstants have been reported earlier. NMR-pH Profiles, macro- and microscopic protonation schemes, and species-specific distribution diagrams are presented. The physicochemical data obtained can help to understand the steric and electronic features governing the selectivity of binding and to design new therapeutic agents.