Syn- and anti-isomers of an adamantylideneadamantane 1,2-dioxetane having a phthalimide side chain were prepd. and investigated their cryst.-state chemiluminescence (CL) properties. The isomers showed contrastive CL properties depending on their crystal-structural characteristics, indicating that CL provides an attractive target for real-time monitoring of a chem. reaction in the crystal. [on SciFinder(R)]