The thermal stability of the infrared stimulated luminescence (IRSL) signal from a sedimentary K-feldspar was investigated using isothermal decay study. It is observed that the isothermal decay of IRSL signal cannot be described using a first-order exponential decay function. Instead, the decay can be well described by considering the presence of band-tail states. Based on the isothermal decay results, a trap depth of ∼1.92 eV was obtained for the IRSL stimulated at 50 °C and the width of the band-tail states was found to be ∼0.37 eV below the conduction band edge. Deeper trap depths (up to ∼2.06 eV) were obtained for multi-elevated-temperature post-IR IRSL (MET-pIRIR) at elevated temperatures. Based on these observations, the implication for the IRSL dating feldspar was discussed.