The Collaborative International Dictionary of English v.0.48:
Incommutability \In`com*mu`ta*bil"i*ty\, n. [L. incommutabilitas: cf. F. incommutabilit['e].] The quality or state of being incommutable. [1913 Webster]WordNet (r) 3.0 (2006):
incommutability n 1: the quality of being not interchangeable