According to Valderrama (1981) aquatic samples are successfully digested with peroxodisulphate prior to total nitrogen determination. This study compares the utilization of a microwave oven and an autoclave for the preparation of liquid samples in terms of precision and accuracy of the obtained recovery. The comparison encompasses both reference solutions such as urea, amino acids, nucleic acids and recalcitrant compounds as well as natural water samples. Although the observed differences are small, the microwave digestion technique proofs significantly more precise. On the other hand, biases introduced by each of the techniques are similar. The slightly but significantly better precision and its easy utilization on board research vessels make the microwave digestion technique highly recommendable. (C) 1998 Elsevier Science Ltd. All rights reserved.