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The ATLAS Muon Spectrometer relies on monitored drift tube (MDT) chambers for precision tracking. In standard conditions, MDTs operate with an Ar 93%-CO2 7% mixture at 3 bar absolute pressure. Any variation of the gas mixture has an impact on the detector performance. During a beam test at CERN in the summer 2002, the drift properties of several production MDT chambers were studied as a function of temperature, composition of the gas mixture, water vapor content, applied voltage and discriminator threshold. A second beam test still at CERN in July 2002 was devoted to studying the performance of MDT chambers in the presence of a high photon background.