The solubility of L-cystine, H2L, as well as the acidic constants have been Studied at 25 degrees C in different ionic strengths. The media were (0.50, 1.00, 2.00, and 3.00) mol.dm(-3) NaClO4. The solubility of H2L was also determined in pure water. The protonation constants of L-cystine, K-n (Hn-1Ln-3 + H+ reversible arrow HnLn-2) and Kn+2 (H2+n-1Ln-1 + H+ reversible arrow H2+nLn), in a basic and in a more acid range, respectively, combined with the salting effects of NaClO4 on H2L deduced from the solubility determinations, were processed by the specific ion interaction theory, SIT, to give equilibrium constants at the infinite dilution reference state, log degrees K-1 = 9.41 +/- 0.05, log degrees K-2 = 8.64 +/- 0.01, log degrees K-3 = 1.61 +/- 0.06, and log degrees K-4 = 1.36 +/- 0.09, as well as specific interaction coefficients, b(L2-, Na+) = 0.29 +/- 0.02, b(HL-, Na+) = 0.14 +/- 0.01, b(H3L+, ClO4-) = 0.15 +/- 0.01, and b(H4L2+, ClO4-) = 0.29 +/- 0.02.