Europium(III) is determined in a flow system based on the reduction of flavin mononucleotide, FMN, by Eu(II) followed by bacterial bioluminescence. Using flow injection analysis, Eu(III) is first reduced to Eu(II) in a zinc amalgam mini-column. The Eu(II) then reduces FMN to FMNH 2 and the bioluminescence produced on aerial oxidation of FMNH 2 in the presence of alkanal mono-oxygenase (FMN linked), EC 1.14.14.3, and n-decanal is used to determine the Eu(II). Calibration is achieved for 5 10 - 7 to 1 10 - 4 M Eu(III) with a limit of detection (3 s.d.) of 5 10 - 7 M and an analysis rate of 60 h - 1 . The effects of possible interferences are reported.