1,8-Cineole synthase (EC 4.2.3.108, 1,8-cineole cyclase, geranyl pyrophoshate:1,8-cineole cyclase, 1,8-cineole synthetase) is an enzyme with systematic name geranyl-diphosphate diphosphate-lyase (cyclizing, 1,8-cineole-forming). This enzyme catalyses the following chemical reaction
This enzyme requires Mn<sup>2+</sup> or Zn<sup>2+</sup>. Geranyl diphosphate first isomerizes to (S)-linalyl diphosphate which ionises to the alpha-terpinyl cation which reacts with water to form the product.