We have investigated the epitaxial regrowth and diffusion during thermal annealing of C- and N-implanted Si and α-quartz, utilising Rutherford Backscattering Spectroscopy in Channeling geometry (RBS-C) and nuclear reaction analysis (NRA). In both materials solid phase epitaxial regrowth was observed to occur at temperatures around 500 o C (Si) and 1000 o C (SiO 2 ). Epitaxial regrowth in Si was found to be strongly retarded compared to self-ion-implantation and the epitaxial recrystallisation front stopped at about the depth of maximum implant concentration even after annealing at 900 o C. In fact, the implanted N-concentration profile in Si was found to be unchanged up to this temperature. This points at a chemical reaction between Si and the implanted C- and N-atoms, which obviously suppresses epitaxial recrystallisation. In SiO 2 almost no nitrogen could be detected in the N-implanted sample at T >= 900 o C, i.e. no significant amount of impurity atoms was present during recrystallisation, which may hinder or suppress the regrowth process. A similar behavior is expected for the implanted C-atoms. In fact, epitaxial recrystallisation in the C-implanted α-quartz sample was found to be almost complete, except for a small amount of defects in the recrystallised matrix.