The Collaborative International Dictionary of English v.0.48:

Undecomposable \Undecomposable\
   See decomposable.

WordNet (r) 3.0 (2006):

undecomposable
    adj 1: representing the furthest possible extent of analysis or
           division into parts; "a feeling is a simple and
           undecomposable mental state"- G.S.Brett; "this weight of
           evidence is something mystical and
           unanalyzable"-M.R.Cohen [syn: unanalyzable,
           undecomposable]