The Collaborative International Dictionary of English v.0.48:

Passionless \Pas"sion*less\, a.
   Void of passion; without anger or emotion; not easily
   excited; calm. "Self-contained and passionless." --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

passionless
    adj 1: not passionate; "passionless observation of human nature"
           [ant: passionate]
    2: 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:

67 Moby Thesaurus words for "passionless":
   affectless, anesthetized, apathetic, arctic, autistic, blunt,
   catatonic, chill, chilly, cold, cold as charity, cold-blooded,
   coldhearted, cool, desireless, detached, dispassionate, drugged,
   dull, emotionally dead, emotionless, frigid, frosted, frosty,
   frozen, heartless, icy, immovable, impassible, impassive,
   inappetent, indifferent, inexcitable, inhibited, insusceptible,
   loveless, nirvanic, nonemotional, objective, obtuse, out of touch,
   self-absorbed, soulless, spiritless, unaffected, unaffectionate,
   unambitious, unaspiring, unattracted, uncaring, unconcerned,
   undersexed, undesirous, uneager, unemotional, unenthusiastic,
   unfeeling, unimpassioned, unimpressionable, unloving, unmoved,
   unpassionate, unresponding, unresponsive, unsusceptible,
   unsympathetic, untouchable