Nanocomposite adsorbents synthesized by in-situ nucleation of hydrous ferric oxide nanoparticles (HFOs) within porous matrices have been extensively studied and applied in polluted water remediation. In this study a series of nanocomposites were developed by loading HFOs inside different porous materials, including anion exchanger (Resin(+)), cation exchanger (Resin(-)), neutral-chloromethyl resin (Resin(0)) and granular activated carbon (GAC), and the stability of the encapsulated HFOs was investigated as a function of pH, ionic strength, temperature and coexisting foreign ions (Fe2+/PO43-). Protonation-induced Fe3+ release was observed for all the tested nanocomposites, and increasing ionic strength of solution facilitated Fe3+ release. HFOs encapsulated in Resin+ phase retarded Fe3+ release as compared to those in Resin(-)/Resin(0) phases. Meanwhile, the HFOs retention capacity increased drastically (e.g., from 20.7% to 75.8% at pH 1.35) with decrease in the average pore diameter of Resin(0) from 32.5 nm to 9.5 nm. Coexisting phosphate not only efficiently suppressed the dissolution of HFOs inside the porous matrices, but hindered their transformation into other crystalline counterparts. Catalytic structural transformation of amorphous HFO phases into thermodynamically more stable phases by Fe (II), commonly observed in bare HFOs systems, was not observed for the encapsulated HFOs. Additionally, EXAFS analyses indicated that the HFOs in Resin-phase have stronger binding energy with Cu (II) ions than the bare HFOs. These results are expected to provide necessary information for the optimization of such nanocomposite adsorbents and their practical applications.