The Collaborative International Dictionary of English v.0.48:

isosmotic \isosmotic\ adj.
   having the same or equal osmotic pressure; isotonic; -- used
   of solutions. Contrasted with hypertonic and hypotonic

   Syn: isotonic.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

isosmotic
    adj 1: (used of solutions) having the same or equal osmotic
           pressure [syn: isotonic, isosmotic] [ant:
           hypertonic, hypotonic]