In this work the time resolved mode mismatched thermal lens (TL) method is applied to investigate Cr(VI) species in water. An abnormal behaviour of the TL transient induced by a photochemical reaction was observed during the sample excitation. The results showed that by using the time resolved method it was possible to discriminate the photochemical reaction processes from the TL effect. The adopted procedure confirms the usefulness of the TL for the study of photochemical reactions with the advantage of monitoring the processes in a quantitative way and with a temporal resolution of the order of a few milliseconds. (C) 2004 Elsevier B.V. All rights reserved.