Deposition of thin layers of fine-grained tephra and acid precipitation can affect large areas in the downwind zone of erupting volcanoes. Due to the rarity and unpredictibality of large-scale volcanic eruptions, evidence for the effects of these events on biological communities and ecosystem processes from field observations is scarce. Furthermore, it is difficult to separate the relative importance of physical and chemical impacts on biota caused by volcanic eruptions. To elucidate the effects of fine-grained tephra and acid precipitation on Sphagnum moss, we conducted a mesocosm experiment using the widespread species Sphagnum palustre. We combined two levels of tephra (no tephra/5 mm of tephra, obtained as commercially available material from the Aira caldera eruption in southern Kyushu, ca. 25000 yBP) with three levels of acidity (neutral, pH 4, pH 2), resulting in six treatments that were replicated ten times. Tephra was applied once at the start of the experiment, while ca. 10 ml of water with different acidity were applied after each height measurement. The height of the Sphagnum palustre capitula was measured in ten locations per mesocosm at the start of the experiment and again after 7, 14, 21, 35, 49, 63, 77 and 91 days using a fixed laser distance meter. The average of the measurements was used to characterize Sphagnum height in the mesocosms. In the first week, height declined by 3 – 4 mm in all treatments. After two weeks, height remained stable in all treatments except for pH 2 without tephra addition where height had declined further by 1 -2 mm. The control and the treatments with pH 4 (both with and without tephra) had increased in height by 22 – 28 mm by the end of the experiment, whereas the treatment with tephra and pH 2 had increased only by 12 mm. Height in the treatment with pH 2 without tephra addition had declined by 8 mm on average. The results suggest that Sphagnum palustre growth was negatively affected only by strongly acidic precipitation (pH 2), whereas pH 4 did not cause significant changes compared to the control. Tephra mitigated the effect of strongly acidic precipitation to some extent. Chemical impact of acidic precipitation may be more relevant than physical effects of thin tephra layers for the growth of Sphagnum mosses after volcanic disturbance.