We present emission channeling experiments on the lattice location of Er in CZ Si single crystals with a well-defined O concentration of 6.5-6.6 x 10(17) cm(-3) and 60 keV-implanted Tm + Er doses ranging from 4.3 x 10(12) to 3.6 x 10(13) cm(-2). The experimental results are compared with the predictions of a simulator which models the formation of ErnOm clusters on the basis of simple diffusion and capture kinetics. We find that our experimental data compare favorably with a scenario where the formation of Er-n O-m defects with one or more O atoms is responsible for removing the Er atoms from their tetrahedral interstitial (T) sites. This suggests that Er does no longer occupy the T site even in simple (ErO) pairs. (C) 1999 Elsevier Science B.V. All rights reserved.