Five mercury (Hg) chemistry models are compared using the same data set for model initialisation. All five models include gas-phase oxidation of Hg(0) to Hg(II) (except for one model), fast reduction–oxidation aqueous reactions between Hg(0) and Hg(II), and adsorption of Hg(II) species to soot particles within droplets. However, the models differ in their detailed treatments of these processes. Consequently, the 48-h simulations reveal similarities but also significant discrepancies among the models. For the simulation that included all Hg species (i.e., Hg(0), Hg(II) and Hg(p)) as well as soot in the initial conditions, the maximum simulated Hg(II) aqueous concentrations ranged from 55 to 148 ng l−1 whereas the minimum concentrations ranged from 20 to 110 ng l−1. These results suggest that further experimental work is critically needed to reduce the current uncertainties in the formulation of Hg chemistry models.