Variations in the molecular aggregation structures and optical properties of a semialiphatic fluorinated 10FEDA/DCHM (poly(4,4'-diaminocyclohexylmethane 1,4-bis(3,4-dicarboxytrifluorophenoxy) tetrafluorobenzeneimide) polyimide (PI), which exhibits strong cyan fluorescence, were examined under very high pressure up to 8 GPa using synchrotron wide-angle X-ray diffraction (WAXD) and fluorescence spectroscopy. The fluorescence intensity of a PI film imidized at 220 degrees C was significantly reduced by applying pressure up to 1 GPa, which should be due to an appreciable reduction in interchain free volume, as indicated by a decrease in the d-spacing values of WAXD peaks which correspond to intermolecular ordering. In contrast, a PI film imidized at 300 degrees C, which exhibited weaker fluorescence than that imidized at 220 degrees C at atmospheric pressure, demonstrated a much smaller reduction in fluorescence intensity below 1 GPa. Such differences in pressure dependence clearly reflect the degrees of PI chain packing formed at different imidization temperatures. These phenomena induced by high pressure were almost reversible between pre- and postpressurization states with small hysteresis in the WAXD patterns and fluorescence spectra.