After oxidation of the surface of cross-linked cis-1,4-polyisoprene with aqueous permanganate, the composition of its interface with water depended on temperature. The polymer remained hydrophilic against cool water but became more hydrophobic against hot water. These changes depended upon the amount of cross-linking and on the amount of surface oxidation. They were also reversible when cycled between high and low temperature, and this reversibility disappeared only gradually over many cycles. The system independence of this behavior provided an important test of our hypothesis that the surface dynamics of elastomers can involve a competition between enthalpically favorable solvation of hydrophilic functional groups at the polymer/water interface and entropically unfavorable chain extension.