Optical properties of Yb3+–Tb3+ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 μm. The Tb3+ ions are excited by means of energy transfer processes from Yb3+ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3+ ions and their dependence on the excitation intensity at about 1 μm has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3+ ions. An efficient backtransfer process is observed from Tb3+ ions towards Yb3+ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix. © 2001 American Institute of Physics.