The Collaborative International Dictionary of English v.0.48:

Thermolysis \Ther*mol"y*sis\ (th[~e]r"m[o^]l"[i^]*s[i^]s), n.
   [Thermo- + Gr. ly`ein to loose.] (Chem.)
   The resolution of a compound into parts by heat; dissociation
   by heat.
   [1913 Webster]