Dehydrins are a family of proteins that accumulate in response to abiotic stresses. Little is known about the biochemical functions of these proteins. It is known that the Arabidopsis dehydrin, ERD14, is activated by phosphorylation to bind calcium and other ions. To begin to categorize the Arabidopsis dehydrins into functional families, we determined whether representative members of the dehydrin sub families share the properties of ERD14. When phosphorylated in vitro with casein kinase II; recombinant COR47, and ERD10 (and ERD14) become activated to bind calcium. ERD14 exhibited the highest calcium-binding activity followed by ERD10 and COR47. These dehydrins, when isolated from cold-treated Arabidopsis plants were also shown to have phosphorylation-dependent, calcium-binding activity. RAB18 showed very little calcium binding activity, even though it was phosphorylated by casein kinase II. XERO2 was not phosphorylated with CKII and did not bind calcium. Competition studies suggest that other divalent cations may bind to the dehydrins COR47, ERD10, and ERD14. Utilizing matrix-assisted laser desorption ionization – time of flight mass spectroscopy (MALDI-TOF), we determined that the poly serine region located in all three calcium-binding family members (COR47, ERD10, and ERD14) is the most likely phosphorylation site responsible for the activation of calcium binding. These results are consistent with a distinct biochemical function for the acidic subclass of dehydrins (COR47, ERD10, and ERD14) as ion (calcium)-interacting proteins.