A new tetradentate beta-diketone chelator, 1,10-bis(8'-chlorosulfo-dibenzothiophene-2'-yl)-4,4,5,5,6,6,7,7, -octafluorodecane-1,3,8,10-tetraone (BCOT), was synthesized. This compound forms a stable chelate with europium (III) and can be used as a fluorescence label for time-resolved fluorescence spectrometry. BCOT can be covalently bound to a protein under relatively mild conditions and, when complexed with europium(III), both in 0.1 M Tris-HCl solution and 1.0 x 10(-5) M tri-n-octyl phosphine oxide (TOPO)-0.05% sodium n-dodecyl sulfate (SDS)-0.1 M NaHCO3 solution, forms a strongly fluorescent chelate having lifetimes of 225 microseconds and 240 microseconds, respectively. As a model reaction, bovine serum albumin (BSA) was labeled with BCOT in a carbonate buffer solution. Fluorescence properties of the labeled BSA solution were studied. A time-resolved fluorescence determination of the BSA(BCOT)40-Eu3+ solution was carried out. Detection limits of 9.3 x 10(-14) M (TOPO-SDS-NaHCO3 solution) and 6.7 x 10(-13) M (Tris-HCl solution) for BSA were obtained by the method.