ISO spectroscopy of ultraluminous infrared galaxies

Spectroscopy from the Infrared Space Observatory ISO has for the first time provided the sensitivity to exploit the diagnostic power of mid-infrared f ine structure lines and PAH features for the study of ultraluminous infrare d galaxies (L-IR > 10(12)L.). We report on observations obtained with SWS, ISOPHOT-S, and the CVF option of ISOCAM. From both fine structure lines and PAH features, we find that the majority of ULIRGs is predominantly powered by star formation. Our total sample of about 75 ULIRGs allows to search fo r trends within the class of ULIRGs: The fraction of AGNs increases with lu minosity above similar to 3 x 10(12)L. but there is no obvious trend for UL IRGs to be more AGN-like with more advanced merger phase.