Abstract. A newly developed diffusing laser applicator was examined for interstitial laser coagulation (ILC) of liver tumours. The applicator consisted of a matted quartz core and a quartz glass dome, also matted on its inner surface and sealed to the fibre. The applicator provided a homogeneous light intensity distribution over an active length of about 20mm. Lesions were created in an ex-vivo porcine liver model using a Nd-YAG laser comparing the new diffusing tip with a Ringmode-ITT applicator in order to find optimal laser parameters and damage thresholds. The lesions were investigated using macroscopic size measurement, volume calculation and histological examination (HE, NADPH-dehydrogenase). The damage threshold of the diffusing tip was 6W at 14min exposure time whereas the Ringmode-ITT applicator had its limit at 5W and 12min exposure. Comparing various exposure times showed that treatment over a time of more than 840s did not significantly increase the lesion volume. At 5W and 720s the mean lesion volume was 6.91.1cm3 with the diffusing tip and 6.30.6cm3 with the Ringmode-ITT applicator, both having a slight ellipsoidal shape. Hence, the created lesions were not significantly different for both applicators when the same laser parameters were applied. On the other hand, the new diffusing tip had a higher damage threshold and was therefore capable of producing maximal coagulation volumes of up to 7.90.5cm3 at 5W and 20min. The experiments showed that lesions with a dimension of 3122mm can be achieved with the diffusing applicator which seem suitably sized for treating small human liver metastases in a single laser session.