From emf values of galvanic cells of the types {Na-glass \ NaCl (m(B)), HZ(+/-) (m(A)) \ AgCl(S) \ Ag}; {glass-H \ HCl (m(C) = 0.01 mol kg(-1)), HZ(+/-) (m(A)), NaCl (m(B)) \ AgCl(S) \ Ag} and {glass-H \ HZ(+/-) (mA), NaZ (m(D) = 0.09982 mol kg(-1)), NaCl (m(B)) \ AgCl(S) \ Ag} (where HZ(+/-) standsfor DL-alpha-amino-n-butyric acid), activity coefficients for NaCl and the aminoacid were determined, at various relative concentrations of both compounds, in the ionic strength range of 0.1-4 mol kg(-1) and 298.15 K. Likewise, activity coefficients for both the pure aminoacid hydrochloride (up to 0.5 mol kg(-1)) and the sodium salt, of the aminoacid (up to 0.1 mol kg(-1)) were also determined, together with the thermodynamic ionisation equilibrium constants for these latter compounds.