The ionic interactions have been investigated from the isopiestic measurements on the quaternary systems H2O-Er(NO3)(3)-La(NO3)(3)-Pr(NO3)(3), H2O-Er(NO3)(3)-La(NO3)(3)-Nd(NO3)(3), and H2O-Er(NO3)(3)-Pr(NO3)(3)-Nd(NO3)(3) at 298.15 K to near saturation. The isopiestic measurements can be represented by a modified Pitzer ion-interaction model extending to C-(3) within the experimental uncertainty over a full concentration range. In addition, the Zdanovskii-Stokes-Robinson model or partial ideal solution model is obeyed by all of the systems within isopiestic accuracy, indicating zero exchange energy between the unlike salts, which is consistent with the nature of rare earth elements.