The Tb (NO3)3-PEO system was characterized to have a solubility limit equivalent to n congruent-to 6.5 and a coordination number of 12.4 +/- 0.1, compared to 5.0 and 10.9 +/- 0.1 for the Nd(NO3)3-PEO system. The greater coordination involving Tb3+ appears to be responsible for the lower asymptotic glass transition temperature at high doping levels of the Th (NO3)3-PEO complex. In both systems, anhydrous amorphous complexes were obtained below the respective solubility limits, over which apparent solubilities were found to be enhanced by the presence of tightly bound moisture while retaining the amorphous character of the complexes. Whereas partial precipitation was observed in the Tb (NO3)3-PEO system upon exposure to ambient moisture, both complexation and amorphous character were found to persist in the Nd(NO3)3-PEO system in the presence of absorbed moisture.