This study provides an experimental evidence of a non-linear state of charge transfer vibronic excitons (CTVE) and its well-defined metastable behaviors in KTaO3. An IR source could induce the same green CTVE-recombination luminescence band as that by a UV pumping. This up-conversion luminescence is characterized by a relaxation time tau (tau similar to 60 s at 3 K), and theoretically explained as a result of IR-induced transfer of occupation of a metastable anharmonic CTVE state into a harmonic luminescent CTVE state. The observed temperature dependence of tau suggests that the relaxation is controlled by resonant and phonon-induced tunneling processes. (c) 2006 Elsevier B.V. All rights reserved.