Source : Webster's Revised Unabridged Dictionary (1913)

Incommutability \In`com*mu`ta*bil"i*ty\, n. [L.
   incommutabilitas: cf. F. incommutabilit['e].]
   The quality or state of being incommutable.

Source : WordNet®

incommutability
     n : the quality of being not interchangeable