We show that attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy can be used to determine the surface charge density (SCD) of colloidal silica nanoparticles (NPs) in aqueous solution. We identify the Si-O stretch vibrations of neutral surface bound silanol, Si-OH, and of the deprotonated group, Si-O-. The position of the Si-(OH) stretch vibration is shown to directly correlate with the NPs SCD as determined by traditional potentiometric titrations, shifting to lower wavenumber (cm-1) with increasing density of Si-O-. The origin of this shift is discussed in terms of inductive effects that reduce the ionic character of the Si-(OH) bond after delocalization of the negative charge left on a terminal Si-O- group across the atoms within similar to 1 nm of the charged site. Using this new methodology, we quantitatively determine the SCD of 9, 14, and 25 nm diameter colloidal silica in varying concentrations of NaCl electrolyte at different bulk pH. This novel spectroscopic approach to investigate SCDs provides several opportunities for in situ coupling, for example, in microfluidic channels or with liquid microjets, and requires only very little sample-all potential advantages over a traditional potentiometric titration.