Isopiestic vapor-pressure measurements were made at 298.15 K for aqueous NaCl + SrCl2 solutions using NaCl(aq) as the reference standard. The measurements for these ternary solutions were made at NaCl ionic strength fractions of y(1) = 0.17066, 0.47366, and 0.82682 for the water activity range of 0.9835 >= aw >= 0.8710. Our results, and those from two previous isopiestic studies, were combined and used with previously determined parameters for NaCI(aq) and those determined here for SrCl2(aq) to evaluate mixing parameters (S)theta(Na,Sr) = (0.0562 +/- 0.0007) kg-mol(-1) and psi(Na,Sr,) = -(0.00705 +/- 0.00017) kg(2)center dot mol(-2) for an extended form of Pitzer's ion-interaction model. These model parameters are valid for ionic strengths of I <= 7.0 mol center dot kg(-1), where higher-order electrostatic effects have been included in the mixture model. If the fitting range is extended to the saturated solution molalities, then S theta(Na),Sr = (0.07885 +/- 0.00195) kg-mol(-1), and psi(Na,Sr,Cl) = -(0.01230 +/- 0.00033) kg(2)center dot mol(-2). The extended ion-interaction model parameters obtained from available isopiestic data for SrCl2(aq) at 298.15 K yield recommended values of the water activities and osmotic and activity coefficients.