The Collaborative International Dictionary of English v.0.48:

Expressionless \Ex*pres"sion*less\, a.
   Destitute of expression.
   [1913 Webster]

WordNet (r) 3.0 (2006):

expressionless
    adj 1: deliberately impassive in manner; "deadpan humor"; "his
           face remained expressionless as the verdict was read"
           [syn: deadpan, expressionless, impassive, poker-
           faced, unexpressive]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "expressionless":
   Olympian, aloof, backward, bashful, blank, chilled, chilly, cold,
   constrained, cool, dead, deadpan, detached, discreet, distant,
   dull, empty, fishy, forbidding, frigid, frosty, glassy, guarded,
   icy, impassive, impersonal, inaccessible, inexpressive,
   inscrutable, introverted, lackluster, lusterless, modest, offish,
   poker-faced, remote, removed, repressed, reserved, restrained,
   reticent, retiring, shrinking, standoff, standoffish, stolid,
   subdued, suppressed, unaffable, unapproachable, uncongenial,
   undemonstrative, unexpansive, unexpressive, ungenial, vacant,
   vacuous, withdrawn, wooden