With the aim to investigate the overall evolution of UIR band features with
hardening of UV radiation (increase of the star's 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).Comment: 4 pages, 3 figures, 1 table, to appear in the Proceedings of the
conference "Planetary Nebulae as Astronomical Tools", edited by R. Szczerba,
G. Stasi\'nska, and S. K. G\'orny, AIP Conference Proceedings, Melville, New
Yor