The in-situ characterization of electrolyte solution/metal electrode interfaces is crucial for an understanding of various electrochemical processes. Here, we propose the referencing method for heterodyne-detected vibrational sum frequency generation measurements that enable determination of the up/down orientation of molecules at electrolyte/electrode interfaces. Proof-of-concept experiments of the proposed method were performed for a self-assembled monolayer of 1-octadecanethiol on a platinum surface in air as well as in water. This study opens a new way to explore the structure and orientation of molecules at buried electrolyte/electrode interfaces.