Mercuric ions influence reactions and intermediates that are involved in forming chlorine dioxide from chlorate ions. Addition of mercuric ions to reaction solutions can aid in understanding the mechanism and kinetics of this system. Mercuric ions do not react with aqueous solutions of chlorine dioxide unless those solutions contain chlorous acid. This unusual effect has enabled us to confirm that chlorous acid is an intermediate in the formation of chlorine dioxide in the methanol-chlorate reaction. This work discusses the effect of mercuric ions on solutions containing various Cl species (chlorine, chlorine dioxide, chlorous acid, and both chlorine dioxide and chlorous acid) and on reactions that involve these same species. The work also explores the methanol-chlorate reaction in the absence of mercuric ions. In the initial stage of the process, chlorine dioxide forms after an induction period, during which an intermediate rapidly forms, maximizes in concentration, and then disappears, and a second intermediate forms and attains a steady-state concentration. We show by several methods, including mercuric ion addition, that the second intermediate is chlorous acid. We have not conclusively identified the first intermediate. The increase and decline in its concentration is accompanied by the development and disappearance of the solution color (yellow). Although the UV absorbance behavior of the system suggests that the intermediate is chlorine, the color of the solution associated with this intermediate is not characteristic of solutions containing chlorine at these concentrations.