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]

The Collaborative International Dictionary of English v.0.48:

Decompose \De`com*pose"\, v. i.
   To become resolved or returned from existing combinations; to
   undergo dissolution; to decay; to rot.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decompose
    v 1: separate (substances) into constituent elements or parts
         [syn: decompose, break up, break down]
    2: lose a stored charge, magnetic flux, or current; "the
       particles disintegrated during the nuclear fission process"
       [syn: disintegrate, decay, decompose]
    3: break down; "The bodies decomposed in the heat" [syn:
       decompose, rot, molder, moulder]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "decompose":
   ablate, anatomize, atomize, break down, break up, canker,
   come apart, consume, corrode, corrupt, crack up, crumble,
   crumble into dust, decay, deliquesce, disintegrate, disjoin,
   disorganize, dissect, dissolve, erode, fall into decay,
   fall to pieces, fester, fission, gangrene, go bad, go off,
   go to pieces, liquefy, melt, mildew, mold, molder, mortify,
   moulder, necrose, putrefy, putresce, rankle, resolve, rot,
   separate, sphacelate, split, spoil, suppurate, taint, take apart,
   turn, turn sour, waste away, wear away