The interactive effects of SO2 and NO3− on the green-algal lichen Ramalina farinacea (L.) Ach. were investigated using the degree of chlorophyll phaeophytinization and ultrastructural damage to the Trebouxia photobiont. Thalli were submitted to several factorial design experiments in which they were exposed to mean SO2 concentrations ranging from 118 to 140 nmol∙mol−1 plus or minus aqueous solutions of NO3− at either 50 or 1000 μmol∙L−1, for 6–14 days. Interactive effects of SO2 and NO3− were synergistic when the effects of each individual pollutant were slight. However, this pattern reverted to additive effects, and sequentially to antagonistic effects as the detrimental impact of SO2 alone became more severe. Despite this transition, neutralization of the damaging impact of SO2 by simultaneous treatment with NO3− was not observed. The results show that interactions between SO2 and NO3− were variable, but predictable on the basis of the expected effects following exposure to each pollutant alone. The coincidence of this empirical pattern with those observed in previous studies suggests involvement of unspecific causes. The wider ecological significance of interactions between SO2 and NO3− is discussed in the light of other findings with respect to the effects of air pollution on lichens in the field. Key words: air pollution, sulphur dioxide, nitrate, interactive effects, lichens, pigments, ultrastructure.