We have analyzed a series of water samples from the perennially ice-covered lakes of the Taylor Valley, Antarctica (lat ∼78°S) for their stable chlorine isotopes (δ37Cl). The three lakes in the valley yielded very different δ37Cl profiles that, when combined with their Cl− variations, can be utilized to ascertain both the source of Cl− to the lakes and their chemical evolutionary histories. The evolution of each lake is closely tied to climatic perturbations within the watersheds that affect the lake's hydrologic balance. Our data indicate that Cl− has been derived from a local marine source (i.e., seawater), marine aerosols, and a longer-traveled HCl source. Lake Bonney may be a remnant of a much older lake system that has undergone filling and drawdown many times, whereas Lake Hoare is probably a relatively new feature (only 1 k.y. old) on the landscape.