With the aim to investigate the overall evolution of UIR band features with hardening of UV radiation (increase of the stars effective temperature) we have analysed ISO spectra for 32 C-rich stars: 20 proto-planetary nebulae and 12 planetary nebulae with Wolf-Rayet central stars. In this contribution we discuss variations in the peak position of UIR bands among analysed objects, and demonstrate that variations in the ``7.7 to 11.3 microns flux ratio are correlated with the effective temperature (probably due to an increase of the ionization state of their carriers).
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