Complexation of tertiary amine containing silica nanoparticles (diameter;: approximate to3.0 nm) and a weak acidic polyelectrolyte, poly(acrylic acid), which has a characteristic pH-responsive property, was investigated under various environments. The salt- and temperature-induced association/transformation/dissociation behavior was studied in aqueous medium by following the turbidity as a function of time. Complex formation was observed in methanol, whereas no complexation was obtained in DMF. The size and density of the aggregate structures formed in methanol could be controlled by the temperature, as confirmed by TEM measurements. Characterization of the isolated complexes was conducted by elemental analysis, H-1 NMR, and FT-IR measurements, suggesting that the silica composition in the complex isolated from water is the same as that obtained from methanol. On the contrary, the kind of solvent has a significant influence on the interaction between the components and the resulting aggregated structures. MALDI-TOF MS was used for the determination of the molecular weights and molecular weight distribution of the silica nanoparticles, which give useful information on the stoichiometry of the complex systems. Morphological and ionization changes of the complexes in response to the environmental changes are discussed in view of the interactions between the two constituents, stoichiometry, and sizes of the cationic particle and the anionic polyelectrolyte.