Solubility and acid-base properties of S-carboxymethyl-L-cysteine (carbocysteine, ccys) in NaCl(aq) and tetramethylammonium chloride, (CH(3))(4)NCl(aq), at t = 25 degrees C and at different ionic strengths were investigated. Solubility was studied at 1.0 <= I(mol L(-1)) < 5.0 for NaCl(aq) and 1.0 < I (mod L(-1)) <= 3.0 for (CH(3))(4)NCl(aq), while potentiometric measurements by 1SE-H(+), glass electrode) were performed at 0.1 <= I (mod L(-1)) <= 5.0 for NaCl(aq) and 0.5 <= I (mod L(-1)) <= 3.0 for (CH(3))(4)NCl(aq). Solubility data allowed us to determine Setschenow constants and activity coefficients of neutral carbocysteine (H(2)ccys). Dependence on ionic strength and ionic medium of protonation constants and activity coefficients was modeled by Specific ion Interaction Theory (SIT) and Pitzer equations. SIT and Pitzer approaches give comparable and satisfactory results. Solubilities and activity coefficients of neutral H(2)ccys species, protonation constants in NaCl(aq) and (CH(3))(4)NCl(aq) at different ionic strengths, SIT interaction coefficients and Pitzer parameters were calculated, together with protonation constants at infinite dilution: log(T) K(l)(H) = p(T)K(a3) = 9.358 +/- 0.001, log(T)K(2)(H) = p(T)K(a2) = 3.461 +/- 0.001, log(T)K(3)(H) = p(T) K(al) = 1.798 +/- 0.004 (in the molar concentration scale, (standard deviation). (c) 2007 Elsevier B.V. All rights reserved.