Considered as one of the most effective methods for debris mitigation, the passivation of spent upper stages is complex to define, implement and qualify. The example of the Ariane 5 EPS is given. After a short description of the selected technical solution, the paper deals with the following points:* safety and reliability aspects linked to the passivation system,* torque resulting from passivation,* behaviour of hypergolic propellants after expulsion and experimental validation,* modelisation of the payload contamination and experimental validation,* thermodynamic behaviour of the propellant at the passivation exhausts and experimental validation,* generation of propellant droplets in the vacuum.