The Collaborative International Dictionary of English v.0.48:

Convertibility \Con*vert`i*bil"i*ty\, n.
   The condition or quality of being convertible; capability of
   being exchanged; convertibleness.
   [1913 Webster]

         The mutual convertibility of land into money, and of
         money into land.                         --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

convertibility
    n 1: the quality of being exchangeable (especially the ability
         to convert a currency into gold or other currencies without
         restriction) [ant: inconvertibility]