Thalli of Xanthoparmelia somloensis with natural content of polyols (control) and polyol free thalli (acetone rinsed) were used to study ribitol effects at low temperatures. Thalli segments were cultivated in ribitol concentration of 32 or 50 mM for 168 h at temperatures +5, 0, and minus 5 deg C. The chlorophyll fluorescence parameters (potential yield of photochemical reactions in PS 2 (variable to maximum fluorescence ratio, Fv/Fm), effective quantum yield of photochemical reactions in PS 2 (Yield PS2), and nonphotochemical quenching (NPQ) were monitored in 24h intervals using an imaging system. The effect of 32 mM ribitol on Fv/Fm and Yield PS2 was apparent only at minus 5 deg C, however, the effect was seen throughout the whole exposure. Surprisingly, 50 mM ribitol concentration treatment led to a decrease in Fv/Fm and Yield PS2 and to an increase in NPQ values at minus 5 deg C, while no change was observed at 0 deg C and +5 deg C. Acetone rinsing caused decrease of Fv/Fm, Yield PS2 and NPQ.