The extraction of inorganic acids such as HCl, HNO3, H2SO4, and HClO4 has been investigated in three neutral phosphorus compound systems, i.e., tri-n-butyl phos phate (TBP), di-isoamylmethyl phosphonate (DiAMP), and trialkylphosphine oxide (TRPO, C-6-C-8), using n-heptane as a diluent. The techniques of electrical conductivity, Fourier transform infrared (FTIR) spectroscopy, H-1 NMR, and dynamic laser scattering (DLS) were selected to characterize the organic phases. The results indicate that the interaction between acids and neutral phosphorus compounds follows a molecule-ion combination mechanism. When the acid in the organic phase reaches a certain concentration, the molecular-ion species have a tendency to self-assemble and form reverse micelles/microemulsions (W/O). The further increase of the acid concentration leads the organic phase to split into two layers. Extractants and acids are mainly located in the down layer, in which aggregations are formed. The tendency to form microemulsions follows TRPO > DiAMP > TBR (C) 2003 Elsevier Inc. All rights reserved.