This solicited ISO open time proposal will use the mid-infrared 'PAH' features to distinguish between starburst and AGN nature in a sample of ultraluminous infrared galaxies (ULIRGs) ISO observations have confirmed previous findings that PAH features are a starburst tracer, while they are weak or absent in classical AGNs. For a sample of about a dozen nearby ULIRGs, PAH spectroscopy with ISOPHOT-S and spectroscopy of high- and low-excitation fine structure lines with SWS agree in the finding that most of them are predominantly starburst-powered. Because of the intrinsic brightness of the PAH emission from star forming regions, the PAH method is ideally suited for extension to more luminous but fainter ULIRGs. The targets for this proposal have been selected to cover the luminosity range 10^12.25-10^12.8 solar luminosities, complementing observations from existing GT and OT programmes and emphasizing high luminosites. Thus, a coherent ISO dataset will be established for which AGN/starburst discrimination is possible on the basis of the extinction-insensitive PAH method. This dataset will allow to probe for changes in the AGN/starburst ratio with ULIRG luminosity, and test evolutionary scenarios.