Photooptical behavior of cholesteric mixtures based on two fractions of nematic polymer with different molar masses containing chiral-photosensitive dopant with C=C double was studied. UV irradiation of planarly oriented films of the mixtures leads to E-Z photoisomerization of dopant, helix untwisting and shift of selective light reflection peak to the longer wavelength spectral region. Kinetics of this process was studied at different temperatures. It was shown that a relative rate of helix untwisting for the mixture with the high molar mass fraction (DP 87) is 2 orders of magnitude less than for the mixture containing a low-molar- mass fraction (DP similar to 22), whereas the activation energies of the helix untwisting for both mixtures have almost the same values. The formation of the ordered TDK* phase during annealing of the blend at 50 degrees C for 1 week completely prevents the helix untwisting. This fact is worthy of attention because it provides a possibility of fixing a helical structure even at temperatures above the glass transition; this advantage makes this system promising for optical data recording and storage.