  Interactions between galaxies have spectacular effects on gas dynamics, and
small galaxy groups are a privileged place to investigate them. In particular,
they could test the existence of cold H2 gas as dark matter in the outer parts
of galaxies.
  HI observations have revealed that galaxies in small groups are deficient in
atomic gas, like in richer galaxy clusters such as Virgo, although in a lesser
extent. Galaxy interactions could be the cause of this deficiency, stripping
the gas out of galaxies and enriching the inter-cluster medium (ICM) in hot
gas, which in turn strips gas through ram pressure. Alternatively, the gas
present at the formation of the group could have been heated to its virial
temperature, and be observed now as X-rays. The dynamical processes related to
this extended gas in small galaxy groups are reviewed.
