The Collaborative International Dictionary of English v.0.48:

Immiscible \Im*mis"ci*ble\, a. [Pref. im- not + miscible: cf. F.
   immiscible.]
   1. Not capable of being mixed or mingled.
      [1913 Webster]

            A chaos of immiscible and conflicting particles.
                                                  --Cudworth.
      [1913 Webster]

   2. Specifically: (Chem.) Not miscible; forming two distinct
      phases when mixed at some concentration; -- of two
      liquids; as, water and gasoline are immiscible.
      [PJC]

WordNet (r) 3.0 (2006):

immiscible
    adj 1: (chemistry, physics) incapable of mixing [syn:
           immiscible, non-miscible, unmixable] [ant:
           miscible, mixable]