Pressure effects on the ionization of various compounds were determined by pressure-modulated fluoresence spectroscopy of buffering solutions containing nanomolar fluorescein as a pH indicator. Two highly pressure-sensitive buffers are described. The second ionization of 1,1-cyclopropane dicarboxylic acid (pK 7.5) has a Delta V-ion of -29 +/- 2 mL/mol, the largest known aqueous ionization volume of any Bronsted acid of pK(a) below 9, The third ionization of 1,4,8,12-tetraazacyclopentadecane exhibits a Delta V-ion of +30 +/- 2 mL/mol, the largest positive aqueous ionization volume known for any Bronsted base in water. The H-1 NMR spectrum of the tetrabutylammonium salt of 1,1-cyclopropane dicarboxylic acid monoanion in acetone-d(6) with 10% H2O at 25 degrees C had a peak at 19.25 ppm, suggesting that the two carbonyl groups are tethered by a low-barrier hydrogen bond which shields electron density from the solvent and that ionization of this proton leads to a large increase in solvation of the carboxylates. The protonation of 1,4,8,1 2-tetraazacyclopentadecane dication causes a structural change that is accompanied by a large increase in charge density and may contribute to the large ionization volume via enhancement of electrostriction.