Optically stimulated luminescence (OSL) signals from feldspar and quartz samples were studied using infrared (860 nm) and green light (420-575 nm) stimulation. A serious problem connected with the regeneration technique used for dating is associated with a change of OSL sensitivity to radiation in the couse of the measurement process. A typical effect seen is a large increase of the apparent strength of our beta source when calibrated against a gamma source. If the regeneration procedure is used, it is shown that the sensitivity increases up to similar to 50% during the measurement process and as a result, the equivalent dose (ED) would be underestimated. A study of sensitivity changes in feldspars and quartz was carried out with emphasis on the effect of preheat and annealing on the OSL signal. Measurement results obtained are presented, and possible elimination of errors in dating caused by sensitivity changes is discussed.