A museum sample of perthitic feldspar was used to study the production of post-IR IRSL signals. It was found that traps responsible for low temperature (similar to 230 degrees C) U peaks play an unexpectedly important role in post-IR IRSL production. During the production of the IRSL signal during low temperature IR stimulation (100 degrees C), electrons are optically transferred from IRSL traps into these TL traps which have been emptied by the preceding preheat at 320 degrees C. Subsequent heating to 300 degrees C causes thermal transfer of these electrons from these traps back into previously emptied IRSL traps which are related to the high temperature U peaks. IR stimulation of these electrons results in post-IR IRSL Thus the initial source of the post-IR IRSL signal is the same as the IRSL signal, with a role being played by intermediate traps that give rise to TL signals between 200 and 250 degrees C, and the final source is similar to that of the IRSL signal. Therefore the post-IR IRSL signal is a by-product of the production of the IRSL signal. It was also found that post-IR IRSL production with high post-IR IR stimulation temperatures (e.g. >230 degrees C) additionally includes a small contribution from the post-IR isothermal decay of high temperature TL peaks that are not sensitive to IR stimulation at low stimulation temperatures.