The individual ionic activity coefficients of Na+, K+, Cl-, and Br- in NaCl, NaBr, KCl and KBr aqueous solutions were determined with individual ion-selective electrode (ISE) at 288.15 K, 298.15 K, 308.15 K, and 318.15 K and salt concentration up to 2m. The experimental results showed that the ion activity coefficients decreased with increasing temperature. At constant temperature, the activity coefficient of Na+ varied with salt concentration and was always larger than that of the counterpart-anions in solution. While the activity coefficient of K+ decreased monotonically and was always lower than that of the counterpart-anion. The activity coefficients of the cation and anion were identical in dilute solution (<0.01m) but different from each other at higher concentrations. The experimental ion activity coefficients were correlated with the modified Pitzer-Debye-Huckel (PDH) model (Pitzer, 1973). The mean activity coefficients of the electrolytes in aqueous solutions were calculated from individual activity coefficients and showed good agreement with the literature data.