With the aim of understanding the fog chemistry in a Chinese megacity, twenty-six fog water samples were collected in urban Shanghai from March 2009 to March 2010. The following parameters were measured: pH, electrical conductivity (EC), ten inorganic major ions (SO42−, NO3−, NO2−, F−, Cl−, Na+, K+, Ca2+, Mg2+, NH4+) and four major organic acids (CH3COO−, HCOO−, C2O42−, MSA). The total ionic concentration (TIC) and EC of fog samples were one or two orders of magnitude higher than those often found in Europe, North America and other Asian countries. Pollutants were expected to be mainly from local sources, including factories, motor vehicle emissions and civil construction. Non-local sources such as moderate- and long-range transport of sea salt also contributed to pollution levels in fog events as indicated by back trajectory analysis. The pH of the fog water collected during the monitoring period varied from 4.68 to 6.58; acidic fogs represented about 30.8% of the total fog events during this period. The fog water was characterized by high concentrations of SO42− (20.0% of measured TIC), NO3− (17.1%), NH4+ (28.3%) and Ca2+ (14.4%). SO42− and NO3−, the main precursors of fog acidity, were related to burning fossil fuels and vehicle emissions, respectively. NH4+, originating from the scavenging of gaseous ammonia and particulate ammonium nitrate and ammonium sulfate, and Ca2+, originating from the scavenging of coarse particles, acted as acid neutralizers and were the main cause for the relatively high pH of fogs in Shanghai. The ratio of (SO42− + NO3−)/(NH4+ + Ca2+) was lower than 1, indicating the alkaline nature of the fog water. A high ratio of NO3−/SO42− and low ratio of HCOO−/CH3COO− were consistent with large contributions from vehicular emissions that produce severe air pollution in megacities.