The apparent dissociation constants of 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS) and 3-(cyclo-hexylamino)-2-hydroxy-1-propanesulfonic acid (CAPSO) 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, dimethyl sulfoxide, acetonitrile, acetone, and dioxane. Initial estimates of the apparent dissociation constant values of the two zwitterionic amino propanesulfonic acid buffers studied have been refined with the ESAB2M computer program. pK(a2)* values change with an increase in the concentration of organic solvent. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents and the possible variation in microheterogenity of the solvation shells around the solute.