The Collaborative International Dictionary of English v.0.48:

Inconvertibility \In`con*vert`i*bil"i*ty\, n. [L.
   inconvertibilitas.]
   The quality or state of being inconvertible; not capable of
   being exchanged for, or converted into, something else; as,
   the inconvertibility of an irredeemable currency, or of lead,
   into gold.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inconvertibility
    n 1: the quality of not being exchangeable; "the
         inconvertibility of their currency made international trade
         impossible" [ant: convertibility]