Monomethylmercury (MMHg) is the compound responsible for the majority of fish consumption advisories in forty eight states in the United States of America. These widespread low level (but toxicologically significant) ecological exposures to MMHg result from microbial methylation of atmospherically deposited inorganic mercury in our national waterbodies. Tributyltin (TBT) is an industrially synthesized antifoulant compound that was historically added to paints applied to the hulls of ships, boats and other watercraft. There is an extensive technical literature on the adverse effects associated with low level ecological exposures to this compound. Although the use of TBT has been restricted in recent years, there exists a significant reservoir of TBT in the sediments underlying our nation's boatyards and marinas. Although there is extensive ongoing research devoted to the behavior of MMHg and TBT in the environment, there does not currently exist a widely available chemical speciation modeling capability for these two toxicants in aquatic media. This report addresses this limitation by initiating a modeling capability for these two species using EPA's geochemical speciation model MINTEQA2. Alterations to the MINTEQA2 components database (COMP.DBS) are documented and a template input file is provided (and documented) that will permit researchers to simulate the aqueous chemical speciation of MMHg and TBT.