An electrochemical cell with a cation and an anion ion-selective electrode (ISE) was used to determine the activity coefficients of two dipeptides, glycylglycine (GG) and glycyl-L-alanine (GA), in aqueous NaCl solutions at 298.15 K. The thermodynamic theory including possible reference states and normalisations of activity coefficients is revisited. The effect of adding dipeptide to the electrolyte solution is stronger at low salt concentration than at higher salt concentration. The change of the activity coefficient ratio of the dipeptide is most pronounced at low concentrations of both solutes. Close to saturation, GA shows salting-in.