Solid‐phase epitaxial growth (SPEG) of silicon was investigated by a tracer technique using radioactive31Si formed by neutron activation in a nuclear reactor. After depositing Pd and Si onto activated single‐crystal silicon substrates, Pd2Si was formed with about equal amounts of radioactive and nonradioactive Si during heating at 400 °C for 5 min. After an 1‐sec annealing stage (450→500 °C in 1 h) this silicide layer, which moves to the top of the sample during SPEG, is etched off with aqua regia. From the absence of radioactive31Si in the etch, it is concluded that SPEG takes place by a dissociation mechanism rather than by diffusion.