In one step further toward developing ATR-FTIR (attenuated total reflection-Fourier transform infrared) spectroscopy as a quantitative technique to interpret interfacial phenomena in aqueous colloidal suspensions, we have used variable angle ATR spectroscopy to concurrently determine the Kramers-Kronig integration constant (or anchor point, n(r)) and spectra of the optical constants (refractive index, n, and absorption index, k) for alpha-FeOOH and TiO2 (anatase) aqueous suspensions. In addition, the absorption coefficient (a) spectra were calculated from the k spectra. The suspension conditions (pH, ionic strength, and concentration) were such that the samples were homogeneous over the sampling depth of the IR radiation, behaving as a thick film for ATR. This study shows that discrepancies in the value of the refractive index of a suspension and its corresponding supernatant generate distortion in the regions of strong water absorption of the ATR difference spectra (suspension spectrum-supernatant spectrum). This distortion reduces the ability of the ATR difference spectra to provide quantitative and in many cases qualitative information on the vibrational characteristics of the particles and associated interface. This information, however, can always be extracted from the a-difference spectra calculated with the methodology proposed in this paper.