  Star formation is evolving very fast in the second half of the Universe, and
it is yet unclear whether this is due to evolving gas content, or evolving star
formation efficiency (SFE). We have carried out a survey of ultra-luminous
galaxies (ULIRG) between z=0.2 and 1, to check the gas fraction in this domain
of redshift which is still poorly known.
  Our survey with the IRAM-30m detected 33 galaxies out of 69, and we derive a
significant evolution of both the gas fraction and SFE of ULIRGs over the whole
period, and in particular a turning point around z=0.35.
  The result is sensitive to the CO-to-H2, conversion factor adopted, and both
gas fraction and SFE have comparable evolution, when we adopt the low starburst
conversion factor of \alpha =0.8 Mo/(K km/s pc^2). Adopting a higher \alpha
will increase the role of the gas fraction.
  Using \alpha =0.8, the SFE and the gas fraction for z=0.2-1.0 ULIRGs are
found to be significantly higher, by a factor 3, than for local ULIRGs, and are
comparable to high redshift ones. We compare this evolution to the expected
cosmic H2 abundance and the cosmic star formation history.
