The structures of water at the interfaces between a fused quartz surface modified by a self-assembled monolayer (SAM) of an amino-terminated silane molecule and electrolyte solutions of various pH and ionic strengths were investigated using sum frequency generation (SFG) vibrational spectroscopy. The structure of the interfacial water strongly depended on the pH of the solution. The effect of the electric double layer generated by the protonation of the amino groups in the SAM and unreacted silanol groups on the quartz substrate at low and high pH, respectively, is considered to be responsible for the pH-dependent SFG behavior.