A water swellable rubber composition comprising (a) a non-water swellable base rubber, (b) an ethylene oxide based hydrophilic elastomer having from zero up to and including 20 mole % of a crosslinkable curable functional group, and (c) a water swellable non-elastomeric material. The composition has excellent water swell characteristics under prolonged exposure to high temperature and various electrolyte solutions (strongly saline as well as acid conditions) and is particularly suitable for oil field applications. When the ethylene oxide based hydrophilic elastomer (b) has from zero to less than 5 mole % of crosslinkable functional group, the amount of (b) is between about 30 parts and 300 parts by weight based on 100 parts by weight of the base rubber (a) and the amount of (c) is between about 20 parts and 200 parts by weight based on 100 parts by weight of the base rubber (a), the composition is characterized by good shape retention after undergoing constrained swell, as well as a high degree of constrained swell at elevated temperatures in different saline concentrations.