Pronounced salt concentration gradients in five antarctic, arctic, and Pacific coastal lakes an be accounted for by diffusional transport of salt out of the deeper saline water layers. The computed values of the mean salt diffusion coefficients, based on the ages of salinity stratification, agree to within an order of magnitude with molecular diffusivities for four out of five lakes. This agreement suggests that no major mixing events occurred in the water column during the late historical stages of the lakes. Upper limit time estimates for the removal of most of the salt from the saline bottom layers range from 5,000 to 35,000 years, depending on lake depth. Historical records of deepening of the Great Bitter Lake owing to dissolution of a salt layer on the bottom suggest that dissolution was a diffusion controlled process. For the saline brines in the Red Sea Deeps, an assumption that they are transient structures leads to the following estimates of the time to mixing with Red Sea water: 103–104 years, if mixing takes place by diffusional transport of salt between the heavier and lighter brines, and 104–105 years, if salt diffuses from the brines upward. The geologically short range of times suggests that the possible recycling of evaporative brines through the deeper ocean could not affect the ocean water salinity for any significant time interval.