Abstract: The invention relates to chemiluminescent compounds of general formula (I) wherein: either: R1 is a reactive group capable of reacting with an amine or thiol moiety; L1 is a hydrocarbon linker moiety comprising 2-12 carbon atoms, optionally substituted with hydroxy, halo, nitro or C1-C4 alkoxy; and R2 is hydrogen, C1-C4 alkyl, C1-C4 haloalkyl, aryl, fused aryl, C1-C4 alkoxy, C1-C4 acyl, halide, hydroxy or nitro; or, alternatively: the combination R1-L1- comprises a C1-C4 alkyl group optionally substituted with hydroxy, halo, nitro or C1-C4 alkoxy; and R2 comprises a group R4-L1-, where R4 is a reactive group capable of reacting with an amine or thiol moiety; and L1 is as defined above; L2 is —C(?O)O—, —C(?O)—S— or —C(?O)N(SO2R5)—, wherein, in each case, the —C(?O) is linked to the ring carbon atom, and R5 is C1-C8 alkyl, aryl, C1-C8 alkoxy or C1-C8 acyl; R3 is a substituted C1-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl or aryl group wherein at least one of the said substituents is electron-withdrawi