The intramolecular cyclisation of 2-hydroxyphenylacetic acid 1 into coumaran-2-one 2 was studied under both microwave irradiation and classical heating for comparison purposes. The use of a monomode oven allowed an accurate consideration of the temperature distribution in the microwave reaction vessel, which revealed a very strong and unexpected thermal heterogeneity. The reaction was facilitated by the presence of a trace of toluene-p-sulfonic acid, the catalytic role of which is demonstrated.