|Proper sealing of the bearing chamber has to be ensured under all operating conditions for safe and reliable operation of aero engines. Oil escaping from the bearing chambers represents an annoyance to the aircraft crew and the passengers and may result in an oil fire risk. This paper presents an experimental study of the sealing behavior of a labyrinth seal typical for aero engine bearing chambers with the presence of a two-phase flow of air and oil. The main objective was to experimentally identify the relevant mechanisms governing the behavior of a leaking bearing chamber seal and to establish a comprehensive database to support the development of optimized analytical methods for bearing chamber design and oil fire prevention. Minimum differential pressures required to prevent oil leakage have been measured for engine relevant operating conditions. In addition, the oil leakage in case of an overpressure in the bearing chamber was quantified. Detailed visualizations allowed resolving the relevant two-phase flow phenomena and characterizing the mixture of air and oil leaving a leaking seal. The experimental programme covered a wide range of rotational speeds, seal clearances, oil flows and differential pressures.