The attenuated total reflectance-Fourier transform infrared (ATR-FTIR) difference spectra of the dilute aqueous (NH4)(2)SO4, Na2SO4, MgSO4, ZnSO4, NaClO4, and Mg(ClO4)(2) solutions by pure water were obtained at various concentrations. In the difference spectra of aqueous (NH4)(2)SO4 solutions, a peak at similar to3039 cm(-1), two shoulders at similar to3155 and similar to2894 cm(-1), and a peak at similar to1445 cm(-1) were ascribed to N-H stretching and bending vibrations, respectively. A small negative peak was resolved at similar to3660 cm(-1) in the difference spectra of (NH4)(2)SO4, which is the sole contribution Of SO42- either in the O-H stretching or in the O-H bending region. The positive peaks of the difference spectra in the O-H stretching region for Na2SO4, MgSO4, and ZnSO4 systems, which constantly appeared at similar to3423, similar to3136, and similar to3103 cm(-1) respectively, were suggested to be the contribution of the interactions between metal cations (Na+, Mg2+, and Zn2+) and water molecules, especially from the first hydrated layer of the cations. In the region of 800-1200 cm(-1), the normally infrared-prohibited nu(1) (SO42-) band was observed as a weak peak at similar to981 cm(-1) even at very dilute concentrations (0.10 mol dm(-3)) due to the disturbance of the water molecules hydrated with SO42-, even though such a feature may increasingly result from associated ions with increasing concentration. The spectra of the water molecules directly influenced by ClO4-, i.e., mostly the first layer of hydrated water, in NaClO4 and Mg(ClO4)2 solutions were obtained by subtracting the corresponding spectra of the same metal sulfate solutions at the same concentrations from the perchlorate solutions. A positive peak at similar to3583 +/- 6 cm(-1) and a negative peak at similar to3184 +/- 25 cm(-1) were obtained as the result of the subtraction. The positive peak was attributed to the water molecules weakly hydrogen-bonded with ClO4-, while the negative one to the reduction of water molecules with fully hydrogen-bonded five-molecule tetrahedral nearest neighbor structure on the introduction Of ClO4-.