The acid - base equilibrium of air - water monolayers of 1-docosyl-4-(4-hydroxystyryl)pyridinium bromide (HSP) on a range of subphases was investigated. On an aqueous subphase the observed pK(a) at half-ionization was found to be considerably lower than that measured in mixed solvent systems. This difference is attributed in part to the enhanced stabilization of the unprotonated form of HSP in the monolayer environment by keto-enol tautomerization. The pK(a) of 11.0 measured for HSP monolayers on dextran sulfate solution was rationalized in terms of the acid group of HSP experiencing a net negative surface potential. Reflection spectra of the HSP monolayer on an anionic cyanine dye solution subphase indicated that only the protonated HSP species is complexed by the dye, giving support to the idea that the unprotonated form lacks a site of appreciable positive charge.