The occurrence and effect of optical absorption of the infrared stimulated luminescence (IRSL) signal and the thermoluminescence (TL) signal from potassium-rich feldspar is investigated for two samples. If optical absorption of the luminescence signal occurs, it is expected that this will cause an age underestimation that will be most severe for coarser grain sizes. Of the two samples under study here, one sample is shown to be affected by optical absorption, whereas the other is not. There is a significant decline in luminescence age with increasing grain size for the sample which exhibits optical absorption, but not for the other sample. However, both samples show a severe IRSL age underestimation for all grain sizes, compared to the age obtained by optical stimulation of quartz from the same samples. We conclude that, although there is an effect of optical absorption on the IRSL age, there must be additional reasons for the underestimation of the IRSL age. (C) 2000 Elsevier Science Ltd.