A collection of water activity data, calculated using the Pitzer model, for Mars-relevant brines. Abstract: Low water activity limits the habitability of aqueous environments, and salts present on Mars are known to reduce water activity. As environmental brines are not pure solutions of a single salt it is difficult to predict their water activity without direct measurement. Martian brines are likely to be complex and dominated by ions including sulfates and perchlorates, unlike typical terrestrial aqueous environments dominated by sodium chloride. We used the Pitzer model to predict the water activity of multicomponent brines and tested against laboratory produced brines, including for the first time perchlorate salts that are known to exist on Mars. Our model matches measurements of single-salt solutions and predicts the water activity of multicomponent brines with an accuracy dependent on the quality of coefficients available for a given ion combination. We tested the hypothesis that some salts will dominate the water activity, and therefore habitability, of multicomponent brines. Some salts, such as sodium and magnesium sulfates, did not strongly modulate the water activity of the solution, whereas others such as magnesium chloride and some perchlorates did. Applied to the history of Mars, the data suggest that sulfates and sodium chloride present in Noachian and early Hesperian environments would not have limited habitability. Perchlorates produced photochemically later in the Amazonian could impose a water activity limit at high concentrations that is not significantly changed by other salts. Overall we found that magnesium and calcium chlorides mixed with perchlorates have low water activity and therefore have the lowest habitability of the brines tested.