A fluorozirconate glass (ZBLAN) containing BaCl2 nanocrystals doped with divalent Eu is a promising material for x-ray image plates for medical diagnosis. Since it is known that Eu2+ readily oxidizes to Eu3+, which reduces fluorescence efficiency of the image plates, 151Eu Mössbauer spectroscopy was used in this work to monitor the Eu oxidation state of the samples during degradation over time in the presence of ambient humidity. In addition, Mössbauer spectroscopic experiments show that the oxidation state has already changed during the glass melt: The sample made from 5 mol% EuCl2 contained 78% EuCl2 + 22% EuCl3 deduced from the relative areas of the absorption lines. The sample made from 2.5 mol% EuCl2 + 2.5 mol% EuCl2 contained 37% EuCl2 + 63% EuCl3, i.e. 26% of the original EuCl2 was oxidized to EuCl3.