According to French standards, constructed wetlands treating combined sewer overflow (CSO CWs) are vertical flow filters with detention basin and outflow limitation. Their purpose is to treat rapid loads of wastewater with stochastic volumes, concentrations and periodicity. The first full-scale CSO CW at Marcy l‘Etoile was monitored to provide in-depth understanding of hydraulics and nitrogen dynamics. Monitoring lasted for three years incl. online equipment. The water content in the media was visualized along the longitudinal section of the filter to follow hydraulics in the fill stage. Tracer tests showed shortcutting at this stage weakening as the filter saturated which tallied with peaks of NH4-N concentrations diminishing at the outflow side. Adverse shortcutting effects can be diminished by minimizing fill time of the media. As for nitrogen dynamics, adsorption capacities showed no difference in the two filter sides, one with a sand-zeolite mixture and the other with pozzolana. An equation was fitted to temperature and adsorbed NH4-N mass measurements to calculate inter-event nitrification. The rate was found to double with every 5.7 °C. The results helped to calibrate the design-support software Orage. Finally, the washout dynamics of NO3-N were analysed to consider the possibility of a second filter stage for denitrification.