[Mitch]

So, the lab has a bottle of Europium that is several decades old. Inside the bottle is just a liquid and the label says its pure >99.99% Europium inside. Does anyone know what compound is formed when Europium is exposed to moisture in the air?

[pantone159]

I think you get Eu2O3 from O2, Eu(OH)3 from H2O, maybe some carbonate, the same as the rest of the lanthanides (except Ce) form. 
I can't imagine how this would turn into a liquid though.  Are you sure this isn't a solution of Eu?  Or the mineral ol/etc it was stored under?  I can't imagine Eu metal lasting decades, it corrodes fine under oil in much less time.

[Mitch]

It was oil, I found the metal ingot in the center.

[jdurg]

Yeah, Europium is one of those pesky little bitches of an element that does everything it can to avoid being sealed away completely oxide free.  Just like strontium and barium.  Take a chunk of the metal and sit it out in an open atmosphere for a few days and you wind up with powdered nothing.