Many random circular flashes of Eu(III) fluorescence (8-23 mum in diameter) appeared at the interface in the extraction of Eu(III) with 2-thenoyltrifluoroacetone (Htta) into dodecane, when a Eu3+ aqueous solution was added after pre-distribution of Htta in the aqueous phase. The flash was caused by the interfacial reaction between a pre-generated micro-aggregate of Eu(tta)(2+) in the aqueous phase and excess Htta in the dodecane phase. The interfacial reaction produced more fluorescent Eu(tta)(3), which diffused away into the dodecane phase.