The Collaborative International Dictionary of English v.0.48:

Commutability \Com*mu`ta*bil"i*ty\, n.
   The quality of being commutable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

commutability
    n 1: exchangeability by virtue of being replaceable [syn:
         replaceability, substitutability, commutability]
    2: the quality of being commutable [syn: commutability,
       transmutability]