The influence of the acid/base properties of Mg2+-modified aluminas on the steady-state and transient behaviour of the ethanol to ethene dehydration has been investigated by simultaneous gas phase and in-situ surface analysis. The steady-state ethanol conversion and the ethene selectivity decrease when the catalyst basicity, i.e., the magnesium concentration, increases. The ethene transient behaviour is characterised by a stop-effect phenomenon, which is a drastic increase of the reaction rate measured for a step-wise decrease in the inlet concentration of ethanol. The relative augmentation of the ethene concentration under dynamic conditions is maximal for the catalyst with a Mg2+/Al3+ atomic ratio of 2.5%. Compared to stationary experiments, transients are much more sensitive to modifications of the catalyst acid/base properties. The experiments were described by a model considering the existence of acid and basic sites on the catalyst surface. The magnesium concentration influences the kinetic parameters, whereas the site concentrations do not show any significant variation.