Apparent molar volumes V-phi and apparent molar heat capacities C-p,C-phi were determined for aqueous solutions of L-proline, L-proline with equimolal HCl, and L-proline with equimolal NaOH at the pressure p = 0.35 MPa. Density measurements obtained with a vibrating-tube densimeter at temperatures (278.15 less than or equal to T/K less than or equal to 368.15) were used to calculate V-phi values, and heat capacity measurements obtained with a twin fixed-cell, differential-output, power-compensation, temperature-scanning calorimeter at temperatures (278.15 less than or equal to T/K less than or equal to 393.15) were used to calculate C-p,C-phi values. Speciation arising from equilibrium was accounted for using Young's Rule, and semi-empirical equations describing (V-phi, m, T) and (C-p,C-phi, m, T) for each aqueous equilibrium species were fitted by regression to the experimental results. From these equations, the volume change Delta(r)V(m) and heat capacity change Delta(r)C(p,m) for the protonation and deprotonation reactions were calculated. Additionally, the Delta(r)C(p,m) expression was integrated symbolically to yield values of the reaction enthalpy change Delta(r)H(m), reaction entropy change Delta(r)S(m), and equilibrium molality reaction quotient Q for both reactions. The results provide a much-improved thermodynamic characterization of aqueous L-proline and of its protonation and deprotonation equilibria. (C) 2003 Elsevier Science Ltd. All rights reserved.