We propose to study the transition between the region of enhanced star formation and the more quiescent disk in the prototypical starburst galaxy M82, by mapping a number of diagnostic emission lines along the major axis. This will provide a characterization of the "super photon-dominated region" that likely marks the transition from the central starburst to the molecular ring and undisturbed parts of M82. M82 represents the unique case of a starburst galaxy suitable for such a spatially resolved study with ISO. We have chosen diagnostic lines, mainly from the 4-12 micron wavelength range offering best spatial resolution, that will allow to 1) Determine the excitation of the ionized medium by observation of lines of [ArII] and [ArIII]. 2) Study emission of several transitions of warm molecular hydrogen, constraining its temperature, and of [SiII] from photon-dominated regions. 3) Determine the temperature and density character of the [FeII] emitting medium, to which HII regions, photon- dominated regions and supernova remnant shocks possibly contribute significantly. During phase 2 proposal entry, the spatial sampling had to be decreased to meet the allocated time. If overheads decrease in the future, this should be restored at least partly to fully achieve the scientific goal.