Thermoluminescence dosimetry (TLD) is performed in many radiotherapy departments using LiF chips which are evaluated by means of a contact heater. The aim of the present study was to investigate the influence of the TL chip position on the heating planchet and to study the self absorption of light in the chips during readout. Experiments were performed using standard LiF chips (3.1 x 3.1 x 0.89 mm3) on a circular heating planchet with a diameter of 13 mm. A two step readout cycle (10s 160 degrees C, 10s 300 degrees C) was used and the integral under the luminescence curve during the second step evaluated. Moving the chip away from the centre of the planchet without adjusting the temperature cycle leads to a peak broadening which can result in a signal loss of up to 40%. The use of a purpose build positioning device tended to improve the reproducibility of the readings. Self absorption of light in a normal chip on the heating planchet was studied by exposing chips in the strong dose gradient of the build up region of a 18 MV X-ray beam. The dose throughout the chip was investigated using a stack of 0.14 mm thick chips. In a 5 x 5 cm2 field the dose varied from 7% at the surface facing the beam to 23% at the opposite side of the chip. Reading the chips with the side facing the beam up in the reader resulted in a 1.9% smaller dose reading than with the chips upside down.(ABSTRACT TRUNCATED AT 250 WORDS)