Cu2+ ions are used to follow the sol-gel transition in amino-modified TEOS mixtures. In contrast to unmodified TEOS gels, fixation of the spin probe (Cu2+) to the sol particle surface takes place and changes in mobility are detectable. Furthermore, we find changes in the coordination sphere of the copper ions and a temporary loss of EPR intensity. By using a simple kinetic model it is possible to estimate the rate constants of the condensation and syneresis processes. (C) 2003 Elsevier Inc. All rights reserved.