The mean ionic activity coefficient (MIAC) ratios of NaCl and NaNO3 in aqueous L-methionine solutions were measured using the potentiometric method. The MIAC ratio of the electrolyte in the aqueous L-methionine solution was reduced at low electrolyte concentrations, while its value increased at higher electrolyte concentrations. In contrast, the activity coefficient ratio of amino acid L-methionine decreased as the electrolyte concentration increased. At higher L-methionine concentration, the MIAC values were higher. Such variations in the activity coefficient ratio at higher electrolyte concentration can be attributed to the formation of an ion-dipole complex (A-(+AA-)C+). The electrolyte- perturbed-chain-statistical-associating-fluid-theory (electrolyte-PC-SAFT) model was used to calculate the MIAC of the electrolyte and activity coefficient of L-methionine in aqueous L-methionine/electrolyte solutions to accurately describe the experimental tendency. The consideration of the effect of an ion-pair complex did not show a significant difference in comparison with results obtained without it, but depicted a better description.