The reactions in fine lines of aluminum and hafnium and the modifications to these reactions introduced by copper-solute additions to aluminum, have been examined as a function of geometry (specifically, linewidth and film thickness) by electrical resistivity, Rutherford backscattering spectroscopy, transmission electron microscopy, and x-ray diffraction. The linewidth varied from submicrometer dimensions (0.5 m) to 200 m. The compound HfAl3 was identified and its growth was found to be diffusion controlled. The temperature dependence of the rate constant for HfAl3 growth from trilayer Al/Hf/Al samples was found to follow Arrhenius behavior from room temperature to 550°C with an average D0 of 3.5 cm2/s [0.034 cm2/s for Al 0.67 wt. % Cu (2 at. % Cu)] and an activation enthalpy of 2.07±0.09 eV (1.79±0.06 eV for Al 0.67 wt. % Cu). Additionally, a second higher-temperature eutectic reaction was found which had a characteristic resistance decrease and associated elemental redistribution. The microstructure of fine lines exhibiting this second reaction was found to be dependent on both sample morphology and geometry. The implications of this second reaction on electromigration behavior and the influence of thermal stress on the chemical potential in driving chemical reactions were considered. It was found that the surface free energy becomes important in fine lines, where the surface-to-volume ratio is large. © 1991 The American Physical Society.