© 2018 The Japan Society for Analytical Chemistry. Micro-organic ion-associate phase (IAP) extraction was combined with a micro-volume back-extraction (MVBE) to reduce coexisting components and viscosity in the concentrates. Heavy metals were converted into a complex with 2-(5-bromo- 2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol in a 40-mL sample solution, and were extracted into ion associates. After centrifugation and discarding the aqueous phase, trace metals were stripped from IAP into a nitric acid solution, followed by GF-AAS determination. Only one vessel was required for 400-fold enrichment. The detection limits (3σ b ) for Cd, Ni, and Pb were 0.6, 3.7, and 0.8 ng/L, respectively. This method was applied in recovery tests in seawater.