  We present a low resolution mid-infrared spectroscopic survey of an unbiased
sample of 62 Ultraluminous Infrared Galaxies (ULIRGs) using ISOPHOT-S on board
ISO. For comparison we also present ISOPHOT-S spectra for 23 AGN and 15
starburst and normal galaxies. The line-to-continuum ratio of the 7.7 $\mu$m
PAH emission feature is used as a discriminator between starburst and AGN
activity in ULIRGs. We find that the majority of ULIRGs are predominantly
powered by starbursts. The ratio L$_{PAH}/$L$_{IR}$ for starburst dominated
ULIRGs is similar to the one found for template starbursts.The shapes of the
PAH features are sometimes unusual.Extinction has a noticeable effect on the
PAH spectra of ULIRG starbursts. We have obtained high resolution near-infrared
imaging for the Southern ISOPHOT-S ULIRGs to investigate their evolution stage.
The majority (68%) of the ULIRGs imaged are double systems and all show
distorted morphologies. Using separations for our work and from the literature,
we have examined whether ULIRGs that are advanced mergers are more AGN-like. We
have found no such evidence contrary to what is postulated by the classical
evolutionary scenario. No correlation is found between the stage of merger in
ULIRGs and their infrared luminosity.We also find that the total mass of
interstellar gas, as estimated from the CO (1-0) luminosity, does not decrease
with decreasing merger separation. When both an AGN and a starburst occur
concurrently in ULIRGs, we find that the starburst dominates the luminosity
output. We propose that the available gas reservoir and the individual
structure of the interacting galaxies plays a major role in the evolution of
the system.
