Between December 2015 and March 2016, fog water was collected at the subtropical mountain cloud forest site Ailaoshan in SW China at 2476 m above mean sea level. An active fog collector was employed to collect 117 samples during more than 140 hours of fog, covering 6 major fog events. The chemical analysis included acidity and inorganic ion concentrations. The median pH values of the fog events varied between 3.7 and 4.2, characterizing the fog water as acidic (pH < 5.0) to very acidic (pH < 4.0). The ion composition was dominated by H+, NH4+, SO42– and NO3–, which made up more than 86% of the total ionic concentration (TIC). The generally rather high ion concentration levels cannot be explained by nearby emission sources but rather by long-range transport of air pollutants from various sources in East Asia. During one event on February 8, 2016, it is evident that it was the emissions from a large coal-fired power plant in Myanmar that led to high concentrations of sulphuric acid. This pilot study of fog chemistry in rural, mountainous SW China should be complemented by studies spanning the moist summer monsoon season and covering more chemical species.