  A method for using the sub-millimeter band to determine photometric redshifts
of luminous high-z dusty galaxies is presented. It is based on the observation
that local ultra-luminous IR galaxies show an average spectral energy
distribution which has a remarkably small dispersion for wavelengths >50
microns. The shape of the long wavelength part is independent on whether the
local ULIRG possesses an AGN or not. A local template is derived from a sample
of ULIRG data presented by Klaas et al. (2001). Using this template and the
internal dispersion in its shape, it is shown that observations at two fixed
submm wavelengths can be used to determine the photometric redshift of galaxies
in the redshift range 1<z<5. The uncertainty in the redshift arising from the
template SED amounts to \Delta z/z ~ 0.3.
