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A strong dependence of thermal activation energy (TAE) on infrared (IR) stimulation time for the infrared stimulated luminescence (IRSL) signal was observed for K-feldspar grains extracted from several sediments and granites from China. A TAE value as low as similar to 0.1 eV was observed at the beginning of IR stimulation and increased to similar to 0.45 eV after 90 s. For a trap depth of similar to 2 eV below the conduction band for the IRSL traps, the TAE value of similar to 0.45 eV is consistent with the energy gap between the excited states (similar to 0.5 eV below the conduction band) and conduction band. This phenomenon is explained as the result of the coexistence of thermally assisted recombination via conduction band or band-tail states hopping and athermal tunnelling recombination of electrons from the excited states under IR stimulation, leading to the observation of a higher anomalous fading rate in the initial part of the IRSL decay curve. (C) 2010 Elsevier Ltd. All rights reserved.