The Collaborative International Dictionary of English v.0.48:

Decompose \De`com*pose"\, v. t. [imp. & p. p. Decomposed; p.
   pr. & vb. n. Decomposing.] [Cf. F. d['e]composer. Cf.
   Discompose.]
   To separate the constituent parts of; to resolve into
   original elements; to set free from previously existing forms
   of chemical combination; to bring to dissolution; to rot or
   decay.
   [1913 Webster]