The thermoluminescence (TL) properties of silicate mineral grains have been used as geochronometers for water-lain sediments. This study provides new insight into the limitations of TL the technique by quantifying the effects of incident wavelength and light intensity on the resetting of the TL signal. The TL residual level increases as wavelengths between 440 and 540 nm are blocked and remains unchanged after ca. 4 hours. These residual levels were maintained even after 24 hours of continuous exposure. The light intensity threshold for reduction in the TL signal is quite low, <10% of full light exposure or <1¿104 joules/m2. However, during the late glacial on Svalbard, and possibly in other aqueous glacial environments, the concentration of suspended sediment was sufficiently high to reduce the ambient light intensities to below these levels, resulting in elevated residual levels that are difficult to duplicate in the laboratory. The available thermoluminescence techniques are unsuitable for unequivocally dating water-lain sediments because of the natural spatial and temporal variations in the solar reduction in the TL signal in turbid aqueous environments. ¿American Geophysical Union 1991