  We present low resolution mid infrared spectra of 16 ultraluminous infrared
galaxies (ULIRGs) obtained with the CVF spectroscopy mode of ISOCAM on board
the Infrared Space Observatory ISO. Our sample completes previous ISO
spectroscopy of ultra- and hyperluminous infrared galaxies towards higher
luminosities. The combined samples cover an infrared luminosity range of
\~10^{12 -13.1} Lo. For about half of the high luminosity ULIRGs studied here,
strong aromatic emission bands suggest starburst dominance. Other spectra are
dominated by a strong AGN-related continuum with weak superposed emission
features of uncertain nature. An improved method to quantitatively characterize
the relative contribution of star formation and AGN activity to the
mid-infrared emission of ULIRGs is presented. As dominant source of the
bolometric luminosity, starbursts prevail at the lower end and AGNs at the
higher end of this range. The transition between mostly starburst and mostly
AGN powered occurs at ~10^{12.4}$ to $10^{12.5} Lo, and individual luminous
starbursts are found up to ~10^{12.65} Lo.
