The Collaborative International Dictionary of English v.0.48:

emotionless \e*mo"tion*less\ adj.
   1. unsusceptible to, destitute of, or showing no emotion;
      unmoved by feeling. Opposite of emotional; as, he kept
      his emotionless objectivity and faith in the cause he
      served. [Narrower terms: matter-of-fact, prosaic;
      philosophical, philosophic; phlegmatic, phlegmatical,
      stolid; stoic, stoical; unblinking] Also See: cool,
      passionless, unmoved(predicate), unmoving.

   Syn: unemotional, passionless.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

emotionless
    adj 1: unmoved by feeling; "he kept his emotionless objectivity
           and faith in the cause he served"; "this passionless girl
           was like an icicle in the sunshine"-Margaret Deland [syn:
           emotionless, passionless]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "emotionless":
   chill, cold, cold-blooded, cool, dispassionate, distant, frigid,
   glacial, heartless, icy, immovable, impassive, impersonal,
   indifferent, matter-of-fact, nonemotional, remote, reserved,
   undemonstrative, unemotional, unfeeling, unimpassioned