We have measured the densities of aqueous solutions of L-methionine, L-methionine plus equimolal HCl, and L-methionine plus equimolal NaOH at temperatures 278.15 <= T/K <= 368.15, at molalities 0.0125 <= m/mol center dot kg(-1) <= 1.0 as solubilities allowed, and at p = 0.35 MPa using a vibrating tube densimeter. We have also measured the heat capacities of these solutions at 278.15 <= T/K <= 393.15 and at the same m and p using a twin fixed-cell differential temperature-scanning calorimeter. We used the densities to calculate apparent molar volumes V-phi and the heat capacities to calculate apparent molar heat capacities C-p,C-phi for these solutions. We used our results and values from the literature for V-phi(T,m) and C-p,C-phi(T,m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change Delta C-r(p,m)(T,m) for ionization of water to calculate parameters for Delta C-r(p,m)(T,m) for the two proton dissociations from protonated aqueous cationic L-methionine. We integrated these results in an iterative algorithm using Young's Rule to account for the effects of speciation and chemical relaxation on V-phi(T,m) and C-p,C-phi(T,m). This procedure yielded parameters for V-phi(T,m) and C-p,C-phi(T,m) for methioninium chloride {H(2)Met(+)Cl(-)(aq)} and for sodium methioninate {Na(+)Met(-)(aq)} which successfully modeled our observed results. Values are given for Delta C-r(p,m), Delta H-r(m), pQ(a) Delta S-r(m), and Delta V-r(m), for the first and second proton dissociations from protonated aqueous L-Methionine as functions of T and m. (c) 2006 Elsevier Ltd. All rights reserved.