We wish to probe the warm ISM in a selection of interacting and merging galaxies. Our goal is to understand better the impact of a nuclear starburst on the surrounding ISM and how the properties of the gas in a galaxy centre relate to the conditions of its disk-gas. Furthermore, we hope to be able to relate the results to the degree and type of interaction. We propose to observe the 9, 12, 17 and 28 micron H_2 lines plus the [C II] 158 micron line in the centre and at least one offset position per galaxy. Because of ISO, we can now, for the first time, directly probe the warm (T>100 K) molecular gas. Starbursts, and galaxy centres in general, are likely to host large amounts of warm molecular hydrogen. In an active starburst, a large fraction of the ISM may exist in the form of photon dominated regions (PDRs). The [C II] 158 and the [O I] 63 micron lines are typical fingerprints of PDRs and are, among other things, useful for assessing gas heating mechanisms. We can use the combined IR lines and their relative intensity to investigate variations in the ISM properties with position in the galaxy, on and off the starburst. We have already an ongoing project of observing and modeling the molecular cloud ensembles in interacting galaxies. We intend to use the ISO IR-data to test and improve our models of the molecular ISM.