  We present a project aimed at studying the structure, dust distribution,
ionization state, and kinematics of a representative sample of 22 ULIRGs. The
galaxies in the sample undergo different merger phases (they are evenly divided
between pre- and post-coalescence systems) and ionization stages (27% HII, 32%
LINER, 18% Seyfert, and 23% mixed classifications) over a wide infrared
luminosity range (11.8<Lir/Lsun<12.6), which also includes some galaxies of low
luminosity. The main aims of this paper are to present the sample and discuss
the structure of the stellar and ionized gas components. Our results imply that
evolution caused by a merger is occurring in the ionized gas structure of
ULIRGs. The present study relies on the use of integral field optical
spectroscopy data obtained with the INTEGRAL instrument at the 4.2 m William
Herschel Telescope.
