The McMurdo Dry Valleys are the largest ice-free area in Antarctica and include several terminal lake basins. These include Lakes Hoare, Fryxell, Bonney, and Vanda. The hydrology of the lakes is controlled by the inputs of glacial meltwater and loss by sublimation of ice covers. Despite the extremely dilute inflow waters, the lakes range in salinity from hypersaline to fresh due in large part to their landscape position and climatic history. Although the aqueous geochemistry of the region has been studied extensively over the past four decades, the ultimate source of solutes to the lakes is still in question. Atmospheric input/precipitation, dissolution of salts in sediments, chemical weathering, remnant seawater, and groundwater inputs have been suggested as the most likely sources of solutes to dry valley waters. We have examined the lithium isotopic composition of glacial snow, stream water, and lake water to better understand the role of these sources in determining the overall chemistry of the lakes. The data suggest that the dominant sources of lithium to the lakes are the dissolution of aeolian dust of crustal origin on the glacial surface, marine aerosols, dissolution of salts and chemical weathering in the stream channels, and ancient seawater. The streams in the valleys show mixing of seawater (and/or marine aerosol) with an isotopically lighter silicate dust end-member as seen in glacial snow. Our data reveal the following sources of lithium to the lakes: streamwater in Lake Hoare; seawater in the monimolimnia (i.e. the dense bottom water in a permanently stratified lake) of Lakes Bonney and Fryxell, a diffusional mixture of Onyx River water and a deep-water brine; hypersaline groundwater discharge in Don Juan Pond. This work confirms, yet provides a more conclusive link than, previous findings between various salt sources and the role they play in solute accumulation in this extreme environment.