The Collaborative International Dictionary of English v.0.48:

Unmask \Un*mask"\, v. t. [1st pref. un- + mask.]
   To strip of a mask or disguise; to lay open; to expose.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unmask \Un*mask"\, v. i.
   To put off a mask. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unmask
    v 1: reveal the true nature of; "The journal article unmasked
         the corrupt politician" [syn: unmask, uncloak]
    2: take the mask off; "unmask the imposter" [ant: mask]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "unmask":
   bare, break the seal, bring to light, debunk, develop, disclose,
   discover, dismask, draw the veil, expose, impart, lay bare,
   lay open, let daylight in, let out, manifest, open, open up,
   raise the curtain, reveal, show, show up, strip bare, uncloak,
   uncover, undrape, undress, unfold, unfurl, unkennel, unpack,
   unroll, unscreen, unsheathe, unshroud, unveil, unwrap