The mean activity coefficients of NaCl in the NaCl-NH4HCO3-H2O system, measured by an electrochemical cell using ion-selective electrodes, were used to calculate the mean activity coefficients of NH4HCO3 in the system through a thermodynamic model. The measurements were made at four temperatures from 293.15 to 308.15 K up to 1.8 mol kg(-1), at NH4HCO3 molality fractions of 0.2 and 0.4. Both sets of values agree with those obtained from Pitzer equation incorporated with chemical reactions. This observation supports the applicability of the existing parameters for calculating mean activity coefficients of this system.