The influence of experimental geometry on infrared total internal reflection surface sum frequency generation (SFG) vibrational spectra at the water/solid interface has been examined. A detailed analysis of the experimental geometry revealed that the enhancement of SFG signal for the "critical angle" can be much weaker than previously thought if the index of refraction of the transmitted or reflected medium is treated as a complex value (i.e., the imaginary part of the index of refraction is not zero and not neglected). The theoretical analysis outlined here agreed well with the experimental results of the SFG spectra of the silica/water interface in two different geometries. This paper deals with the SSP polarization combination.