The chemical phenomenology which occurs in polystyrene films bombarded with low doses 100 keV He+ions has been investigated by infrared spectroscopy. The results obtained show that the polymer is rapidly transformed into a different chemical compound consistent with high cross-linking as indicated by low solubility in CHCI3. The I.R. finding is in agreement with XPS findings and indicates that the observed phenomenology occurs via a mechanism which eliminates the electronic “π” system of the phenyl ring and gives rise to alkene-type bonding.