We have studied deliquescence on the (001) face of single crystals of NaCl and find that it is a nucleated phenomenon. The phase transition occurs only after the relative humidity exceeds that found over the saturated solution by at least 5%. The contrast between our observations and previous measurements using salt powders or crystallites leads us to the conclusion that deliquescence is controlled by the differences in energy required to solvate ions from the smooth (001) face and from the defect-rich surfaces of particulate samples.