We report infrared reflectance studies in the laboratory that reveal the state of hydrogen peroxide on Europa. Detailed analysis of the 3.5 ¿m infrared absorption band for hydrogen peroxide grown in a number of water ice environments shows that, for temperatures and concentrations relevant to Europa (80--130 K), hydrogen peroxide molecules exist in isolated trimers H2O2•2H2O. If the temperature on an icy body containing H2O2 approaches 150 K, as on Ganymede, the trimers aggregate into precipitates of crystalline dihydrate H2O2•2H2O.