Using perturbed angular correlation spectroscopy, jump frequencies of Cd tracer atoms were measured for 12 indides In3(B) (B = rare earth) in paired samples having compositions at each of the opposing phase boundaries. Jump frequencies in heavy lanthanide indides were observed to be smaller for In-richer compositions than for In-poorer compositions, but greater in the light lanthanide indides. These findings signal an unmistakable change in diffusion mechanism from the simple In-sublattice vacancy mechanism for heavy lanthanides to a B-vacancy mechanism for light lanthanides.