Oxaloacetic acid undergoes successive deprotonations to give the dianion:
:HOCC(O)CHCOH     OCC(O)CHCOH  +  H,  pK = 2.22
:OCC(O)CHCOH     OCC(O)CHCO  +  H,  pK = 3.89
At high pH, the enolizable proton is ionized:
:OCC(O)CHCO     OCC(O)CHCO  +  H,  pK = 13.03
The enol forms of oxaloacetic acid are particularly stable.  Keto-enol tautomerization is catalyzed by the enzyme oxaloacetate tautomerase. trans-Enol-oxaloacetate also appears when tartrate is the substrate for fumarase.