Abstract: A novel chemiluminescent labeling compositions comprising an ester or thiolester covalently and jointly bonded to (1) a carbon of a heterocyclic ring or ring system that is susceptible to attack by peroxide or molecular oxygen and (2) an aryl ring or ring system wherein the heterocyclic ring or ring system is distinquished by a heteroatom thereof in an oxidation state which causes the attacked carbon atom to form an intermediate that decays and produces chemiluminescence; the aryl ring or ring system contains at least three substituents on a six-member aromatic hydrocarbon that together sterically and electronically hinder hydrolysis of the linkage, which substituents involve ortho substituent groups on the aryl in conjunction with quaternary ammonium meta or para substituents thereon.