The FV/FM ratio of Lobaria pulmonaria with decreasing pH in samples from a boreal forest influenced by acid rain in SE Norway, suggesting that low pH has an effect on the optimal quantum yield of PSII. The pH was measured of distilled deionized water in which lobes from different habitats were rinsed, probably indicating the ionic environment of the lichen. The best prediction of FV/FM was found in a multiple regression model including the explanatory variables: pH, a/b-ratio and chlorophyll degradation measured as the 666/666a ratio, all correlated positively with FV/FM but not with each other. The commonly used chlorophyll degradation 435/415 ratio cannot be used for L. pulmonaria because of an unidentified, habitat-influenced brownish pigment with high absorbance in the 415–435 nm wavelength range.