The ability to determine hypoxia in tumors in vivo could provide useful diagnostic contrast information. MRI contrast agents could provide this information. Eu2+ is isoelectronic with Gd3+, and produces T1w contrast on a similar level, in addition, it is not oxidatively stable. In an aerobic atmosphere, it oxidizes rapidly to Eu3+, which does not produce T1 contrast, but can belong to a conceptually different class of contrast agents: paraCEST (chemical exchange saturation transfer) agents. A Eu2+/3+ agent was tested in different tissues in-vivo to show the correlation between the rate of oxidation and the surrounding oxidative environment of the tissue.