We present a study of the composite supernova remnant G0.9+0.1 based on observations by XMM-Newton. The EPIC spectrum shows diffuse X-ray emission from the region corresponding to the radio shell. The X-ray spectrum of the whole Pulsar Wind Nebula is well fitted by an absorbed power-law model with a photon index Gamma ~ 1.9 and a 2-10 keV luminosity of about 6.5 X 10^34 d^2_10 erg s^-1 (d_10 is the distance in units of 10 kpc). However, there is a clear softening of the X-ray spectrum with distance from the core, which is most probably related to the finite lifetime of the synchrotron emitting electrons. This is fully consistent with the plerionic interpretation of the Pulsar Wind Nebula, in which an embedded pulsar injects energetic electrons into its surrounding region. At smaller scales, the eastern part of the arc-like feature, which was first revealed by Chandra observations, shows indications of a hard X-ray spectrum with a corresponding small photon index (Gamma=1.0 +- 0.7), while the western part presents a significantly softer spectrum (Gamma=3.2 +- 0.7). A possible explanation for this feature is fast rotation and subsequent Doppler boosting of electrons: the eastern part of the torus has a velocity component pointing towards the observer, while the western part has a velocity component in the opposite direction pointing away from the observer.
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