  More than 150 galaxies have been detected in blank-field millimetre and
sub-millimetre surveys. However the redshift distribution of sub-mm galaxies
remains uncertain due to the difficulty in identifying their optical-IR
counterparts, and subsequently obtaining their spectroscopic emission-line
redshifts. In this paper we discuss results from a Monte-Carlo analysis of the
accuracy with which one can determine redshifts from photometric measurements
at sub-millimetre-FIR wavelengths. The analysis takes into account the
dispersion in colours introduced by including galaxies with a distribution of
SEDs, and by including photometric and absolute calibration errors associated
with real observations. We present examples of the probability distribution of
redshifts for individual galaxies detected in the future BLAST and
Herschel/SPIRE surveys. We show that the combination of BLAST and 850um
observations constrain the photometric redshifts with sufficient accuracy to
pursue a program of spectroscopic follow-up with the 100m GBT.
