The codiffusion of As and B implanted in Si at high doses (1×1016cm−2) and with energies corresponding to the same projected range has been investigated at 900 and 1000 °C on the basis of dopant and carrier profile measurements. The comparison of the codiffusion data with the corresponding ones obtained by the diffusion of each element alone revealed some anomalous effects which can be explained by assuming the formation of neutral donor–acceptor pairs. These complexes are mobile with a diffusion coefficientDpair=17 exp(−4/kT) cm2/s, very close to the diffusion coefficient of As in intrinsic Si. Electrons are the majority carriers in the region where both dopants are present at high densities. On the basis of these features, a diffusion model that takes pairing into account is presented. A simulation program including this model allows one to foresee the anomalous phenomena occurring in the high‐concentration codiffusion of donors and acceptors in Si and in general shows a good agreement with experimental profiles. ©1995 American Institute of Physics.