The mesoscale air quality Mesoscale Nonhydrostatic Chemistry (Meso-NH-C) model is applied to a complex pollution episode over Western Europe during the period 11 to 12 August 1997. As observed in satellite pictures and as simulated, the complexity of this episode is related to the presence of anticyclonic clear-sky areas and regions with deep convective activity in the simulation domain. A brief presentation of the model is made that covers in particular the on-line coupling capability for calculating meteorological and chemical concentration fields at each time step. Then, emphasis is put upon the simulation of transboundary pollution fluxes from London to northern France in a zone of large horizontal wind gradients. Comparison with data from the French Agence De l'Environnement et de la Maitrise de l'Energie (ADEME) pollution network indicates that ozone concentrations and time of arrival of the pollution plume are correctly predicted at surface stations in northern France. A sensitivity analysis relying upon local ozone production and pollution transport has shown that ~30% of ozone maxima levels could be attributed to regional transboundary fluxes.