We propose a program of observational studies using the ISO SWS
that will address the question of what powers the ultraluminous
infrared galaxies.  These objects,discovered in the IRAS all sky
survey, represent a new class of sources having the luminosity of
quasars, and are more numerous than "classical" quasars.  We
propose to study line ratios of the infrared fine structure lines
of [NeII], [NeIII] and [NeV] to provide constraints on the
possible presence of an embedded Active Galactic Nucleus. This
program will distinguish between starbursts and dust enshrouded
quasars as the ultimate power source of these objects and thus
address the question of whether there is an evolutionary
connection from ultraluminous infrared galaxies to optical/UV
quasars.