The chemical composition of nighttime urban fog has been investigated using a hybrid kinetic and equilibrium model. Extremely high acidity may be imparted to the droplets by condensation and growth on acidic condensation nuclei or by in situ S(IV) oxidation. Important oxidants of S(IV) were found to be O2 as catalyzed by Fe(III) and Mn(II), H2O2, and O3. Formation of hydroxymethanesulfonate ion (HMSA) via the nucleophilic addition of HSO3 to CH2O(I) significantly increased the droplet capacity for S(IV) but did not slow down the net S(IV) oxidation rate leading to fog acidification. Gas phase nitric acid, ammonia, and hydrogen peroxide were scavenged efficiently, although aqueous phase hydrogen peroxide was depleted rapidly by reduction with S(IV). Nitrate production in the aqueous phase was found to be dominated by HNO3 gas phase scavenging. Major aqueous phase species concentrations were controlled primarily by condensation, evaporation and pH.