Ion exchange, in which an in-diffusing ion replaces a lattice ion, has been widely exploited as a synthetic tool for semiconductor doping and solid-to-solid chemical transformations, both in bulk and at the nanoscale. Here, we present a systematic investigation of cation-exchange reactions that involve the displacement of Mn2+ from CdSe nanocrystals by Cd2+ or In3+. For both incoming cations, Mn2+ displacement is spontaneous but thermally activated, following Arrhenius behavior over a broad experimental temperature range. At any given temperature, cation exchange by In3+ is approximately 2 orders of magnitude faster than that by Cd2+, illustrating a critical dependence on the incoming cation. Quantitative analysis of the kinetics data within a Ficks-law diffusion model yields diffusion barriers (E-D) and limiting diffusivities (D-0) for both incoming ions. Despite their very different kinetics, indistinguishable diffusion barriers of E-D approximate to 1.1 eV are found for both reactions (In3+ and Cd2+). A dramatically enhanced diffusivity is found for Mn2+ cation exchange by In3+. Overall, these findings provide unique experimental insights into cation diffusion within colloidal semiconductor nanocrystals, contributing to our fundamental understanding of this rich and important area of nanoscience.