The TRANSnational TUNnel operational CBRN risk mitigation (TRANSTUN) project has been able to create a Network of stakeholders coming from more than 15 countries worldwide, draft a set of Operational Guidelines based on a tailored Risk assessment, and undertake a real-life exercise, involving more than 250 first responders from France and Spain, to increase the interagency and inter-country capacity of emergency response in case of CBRN events in real cross-border tunnels. Despite the high costs and the time required to organize a real-life exercise, only this kind of exercises allow to verify and test current operational procedures carried out by rescue teams, first responders and tunnel operators to deal with a chemical accident/event in a tunnel under stressful conditions. For these reasons, real-life exercises also represent a unique opportunity to highlight the gaps that emerged and the critical issues that need to be managed in the response to this type of event. In this work, the findings of the real-life exercise activity performed within the TRANSTUN project at the Bielsa-Aragnouet tunnel are presented. The main objective of this work is to analyse the real-life exercise performed on September 9, 2021 and lasted about 6 hours with the aim to assess safety and security procedures, optimal use of CBRN equipment, knowledge of decontamination processes and finally to evaluate the performance gains and operational capacity by measuring intervention time and the number of personnel required, comparing the different available technologies. The results of the evaluation process carried out at the end of the real-life exercise are reported in this study both in terms of quantity and quality (lessons learned and good practices) and represent an example of a testing methodology that can be used as a reference point for the management of similar activities in the future involving further cross-border tunnels as well.