Er3+-doped silica glass (up to 10 wt%) was synthesized by sol-gel processing with the addition of 3- aminopropyl trimethoxysilane (APS) as a complexing agent. Er3+ ions reacted with amino groups and, thus, linked to the silica network during the sol preparation. As a result, the motion of Er3+ ions was restricted and the formation of Er3+ clusters was inhibited. Both fluorescence spectra and magic-angle spinning (MAS) nuclear magnetic resonance (NMR) indicated that the addition of the complexing agent APS resulted in a homogeneous dispersion of high-level Er+3 doping in the resultant gels. After the removal of organic components, however, Er+3 clustering occurs when firing at a high temperature for a long period of time, e.g. at 1000°C for 10 hrs, due to enhanced Er3+ diffusion.