The effect of compression on the fluorescence and Raman spectra of poly(p-phenylene benzobisoxazole) (PBO) fibre and a PBO/amorphous nylon blend has been investigated qualitatively at various excitation wavelengths. A substantial red-shift in the fluorescence maxima is observed upon increasing pressure both in PBO fibres as well as in the PBO/amorphous nylon blend. The shift is consistent with that observed for aromatic molecular crystals. This observation strongly suggests that certain features in the fluorescence emission are due to interchain interactions.