Abstract In this paper, we studied the acidity of several mordenite samples, which were modified to have 1% (HM1F) and 10% (HM10F) in fluorine w/w, through their catalytic behaviour in the isomerisation of styrene oxide to phenylacetaldehyde in liquid phase. The catalytic experiments were performed by using microwaves as a new method and with a conventional heated batch reactor for comparison. We detect new stronger acid sites for sample HM1F by using NH 3 TPD and FTIR techniques, whereas for sample HM10F some dealumination, and therefore, loss of Brönsted acidity was observed by 29Si, 27Al NMR and X-ray fluorescence. Catalyst HMF1 is the most active at batch reaction conditions, but undergoes faster deactivation when using microwaves. Fluorination in low amounts gives to accessible stronger Brönsted acid sites due to the fluorine located in the external structure framework. This explains the higher activity in the batch reactor and the acceleration of condensation and coke products formation in microwaves experiments. When methanol is used as a solvent, the epoxide ring of styrene oxide opens catalysed by Lewis and Brönsted acid sites. In this case, the existence of high amounts of Lewis acid sites for sample HM10F explains its highest activity.