  Galaxy clusters are not iso-thermal, and the radial temperature dependence
will affect the cluster parameters derived through the observation of the
Sunyaev-Zeldovich (SZ) effect. We show that the derived peculiar velocity will
be systematically shifted by 10-20%. For future all-sky surveys one cannot rely
on the observationally expensive X-ray observations to remove this systematic
error, but one should instead reach for sufficient angular resolution to
perform a deprojection in the SZ spectra. The Compton weighted electron
temperature is accurately derived through SZ observations.
