Coal combustors remain the major uncontrolled anthropogenic source of mercury due to an incomplete understanding of its emission chemistry. Using a new approach, this has now been resolved for the first time. Unexpectedly, mercury is seen to have a propensity for heterogeneous chemistry, previously considered implausible. In systems containing sulfur, it can efficiently deposit as mercuric sulfate. If sulfur-free, mercuric oxide forms. These then are subject to attack by gaseous hydrogen chloride with conversion to gaseous mercuric dichloride. This is the elusive, elegant and dominant mechanism, long overlooked, that can efficiently convert atomic mercury to its controllable water-soluble dichloride. (C) 2004 Elsevier B.V. All rights reserved.