Compact Groups are dense environments where galaxies are strongly interacting and have been shown to experience accelerated evolution from late-types, with copious star-formation, to early-type disk systems. I will present recent results from a Spitzer Space Telescope mid-infrared spectroscopy study of a sample of 23 Hickson Compact Groups; here we find evidence for excess excited molecular hydrogen emission in galaxies nearing the red sequence (in mid-infrared colour and specific star formation). What mechanism is responsible for heating the H2 and is shock-excited molecular hydrogen an important clue to evolution in less dense environments?