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The existence of liquid water is a prerequisite for habitability. Deliquescence of perchlorates under subsaturated conditions has been proposed to explain the occurrence of liquid water in some hyperarid environments on the earth and on the Mars. However, the amount of liquid water associated with perchlorates under different conditions is not well understood yet. In this work, we have measured deliquescence relative humidity (DRH) of three perchlorates at different temperatures from 278 to 303 K. DRH decreases from (42.8 +/- 0.6)% at 278 K to (40.5 +/- 0.5)% at 303 K for Mg(ClO4)(2)center dot 6H(2)O, and from (18.5 +/- 0.5)% at 278 K to (15.5 +/- 0.5)% at 303 K for Ca(ClO4)(2)center dot 4H(2)O; in contrast, deliquescence of KClO4 did not occur even when RH (relative humidity) was increased to 95%. In addition, we have determined the amount of water taken up by Ca(ClO4)(2)center dot 4H(2)O and Mg(ClO4)(2)center dot 6H(2)O as a function of RH (0-90%) and temperatures (278-298 K). It is found that when both salts are deliquesced, more water (similar to 10% on average) is associated with Mg(ClO4)(2)center dot 6H(2)O than Ca(ClO4)(2)center dot 4H(2)O on the per mole ClO4- base. Overall, this work would significantly improve our knowledge in hygroscopicity of perchlorates, and thus may provide fundamental insights into the hydrologic cycles in some hyperarid regions on the earth and on the Mars.