[en] In order to identify the chemical modification along nuclear tracks in CR-39 detectors, we have made a series of FT-IR measurements for thin CR-39 plastic films irradiated by Fe ions. The films were reduced in the thickness by long time chemical etchings down to 5μm from as-received CR-39 sheets of 100μm thick. It enabled us to obtain unsaturated IR spectra. The samples were exposed to 147 MeV Fe ions at HIMAC in air. Amount of loss of carbonated ester bonds due to the exposure was assessed from the changes in the absorbance of C=O and C-O-C bonds with Fe fluence. The assessed G-value for destroying carbonate ester bonds was found to be about 10 (scission/100 eV)