The effects of adding a third transition metal element, Co, Cu, Zr, or Ta, to Al-Nd (or Gd) alloy thin films were studied in order to investigate the changes in the film's microstructure and resistivity, and also their tendency to form hillocks or whiskers at high temperatures. The addition of Zr (or Ta), which has repulsive interactions with both Nd and Gd, to the Al-RE binary alloy films causes an increase of residual resistivity and enhances the formation of hillocks or whiskers at high temperatures. On the other hand, the addition of Cu or Co, which has attractive interactions with both Nd and Gd, results in a resistivity of less than 6 µωcm (depending on the content of added elements). Addition of either of these elements largely suppresses the growth of thermal defects after annealing at over 350 °C. The above results were explained by employing a theory of grain boundary segregation based on a chemical interaction between constituent elements of an alloy.