  Using Herschel PACS and SPIRE observations as part of the HerMES, we explore
the far-IR properties of a sample of mid-IR selected starburst dominated
ultra-luminous infrared galaxies (ULIRGs) at z ~ 2. We derive robust estimates
of infrared luminosities (L_IR) and dust temperatures (Td) of the population
and find that galaxies in our sample range from those that are as cold as
high-z sub-millimeter galaxies (SMGs) to those that are as warm as optically
faint radio galaxies (OFRGs) and local ULIRGs. We also demonstrate that a
significant fraction of our sample would be missed from ground based (sub)mm
surveys (850-1200{\mu}m) showing that the latter introduce a bias towards the
detection of colder sources. Similarly, based on PACS data as part of the PEP
project, we construct for the first time the full average SED of a sub-sample
of infrared luminous Lyman break galaxies at z ~ 3, and find them to have
higher T_d when compared to that of SMGs with comparable L_IR. We conclude that
high-z ULIRGs span a wide range of dust temperatures, larger than that seen in
local ULIRGs, and that Herschel data provide the means to characterize the bulk
of the ULIRG population, free from selection biases introduced by ground based
(sub)mm surveys.
