We have investigated the thermally induced recrystallization of silicon amorphized by sodium-ion irradiation and the diffusion of the implanted Na atoms. Si(100) wafers were irradiated at 77 and 300 K with 50-200 keV Na+ ions to fluences of 10(15)-10(17) cm(-2) and then furnace annealed in the temperature range of 473-943 K. The recrystallization of the amorphous Si and the migration of Na were investigated by Rutherford backscattering spectrometry combined with channeling and by resonant nuclear reaction analysis. At a Na fluence of 1 x 10(15) cm(-2) annealing results in fast and almost complete solid-phase epitaxial regrowth (SPEG) whereas a competition between epitaxial and polycrystalline recrystallization was observed for fluences at or above 10(16) cm(-2). The transition from epitaxial to polycrystalline regrowth, observed when the recrystallization front has reached a depth of high Na concentration, was attributed to the retardation of the SPEG due to pronounced Na segregates located near the amorphous/crystalline interface. During further annealing these segregates then dissolve by grain boundary diffusion of Na towards the surface through the polycrystalline layer. At the critical temperature T-c= 885(30) K, half of the implanted Na content has diffused out of the sample. Hydrogen was observed to be trapped in the amorphized region during annealings at temperatures of 673-723 K, while above it is again released.