The Collaborative International Dictionary of English v.0.48:

Incommutable \In`com*mut"a*ble\, a. [L. incommutabilis: cf. F.
   incommutable. See In- not, and Commutable.]
   Not commutable; not capable of being exchanged with, or
   substituted for, another. --Cudworth. --
   In`com*mut"a*ble*ness, n. -- In`com*mut"a*bly, adv.
   Incompact

WordNet (r) 3.0 (2006):

incommutable
    adj 1: not subject to alteration or change [ant: commutable]
    2: not interchangeable or able to substitute one for another; "a
       rare incommutable skill"

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "incommutable":
   changeless, constant, immutable, inconvertible, indefeasible,
   inert, insusceptible of change, intransmutable, invariable,
   irretrievable, irreversible, irrevocable, lasting, noble,
   nonreturnable, nonreversible, permanent, reverseless, unalterable,
   unalterative, unaltered, unchangeable, unchanged, unchanging,
   undeflectable, undeviating, unmodifiable, unremitting,
   unrestorable, unreturnable, unsusceptible, unvariable, unvarying