Vinyl mercaptoacetate S‐sulfate sodium salt (VMAS) was prepared with a yield of 74% from a reaction of vinyl chloroacetate and sodium thiosulfate. Polymerization of VMAS with 2‐hydroxyethyl acrylate (HEA) in the presence of ammonium persulfate and N,N,N ′,N ′‐tetramethylethylenediamine afforded a colorless copolymer bearing the S—SO3– Na+ group in 3.2–4.3 mol‐% and having a MW of 2.4–13×104. The polymer was water‐soluble but, upon treatment with an oxidizing agent, was transformed quickly into a water‐tolerant material. It again became water‐soluble on the addition of a reducing agent. The unique solubility alternation was explained by the formation and fission of S—S bond crosslinking in the polymer.