The apparent dissociation constants of 3-[N-(tris(hydroxy methyl)methyl)amino]-2-hydroxypropane sulfonic acid (TAPSO), 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropane sulfonic acid (DIPSO), and 3-[N-morpholino]-2-hydroxypropane sulfonic acid (MOPSO) were determined at (25.00 +/- 0.10)degrees C and I = 0.1 mol dm(-3) (KNO3) by potentiometric pH titration in pure water and different hydroorganic solvent media. The organic solvents used were methanol, ethanol, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, acetone, and dioxane. pK(a2)* changes with an increase in the concentration of organic solvent. These results are discussed in terms of solvent characteristics. Solvent basicity and stabilization of the conjugate acid free base by hydrogen-bonding interactions in hydroorganic solvent media relative to pure aqueous media as well as proton-solvent interaction play an important role in the dissociation equilibrium of the N-substituted 3-amino-2-hydroxypropanesulfonic acid buffers studied.