The surface plasmon resonance (SPR) and Schottky effects are important photocatalytic activity boosters in metallic cocatalyst/photocatalyst systems, but it is difficult to differentiate them. In this report, we design a simple method to distinguish the two effects by utilizing a distancetunable self-assembled monolayer (SAM) in a gold (Au)−Cu₂O composite in conjunction with UV and visible-light sources, by which we had only the SPR or Schottky effect identified in the visible or UV light, respectively. Cysteine (cys) and mercaptoundecanoic acid (MUA) SAMs as linkers were used respectively for making Au−cys−Cu₂O and Au−MUA−Cu₂O composites. Au−citrate−Cu₂O as a mild linker was also synthesized. Under UV-light irradiation, Au−Cu₂O showed only the Schottky effect, while Au−MUA−Cu₂O and Au−cys−Cu₂O showed neither of the two effects. Under visible-light irradiation, Au−MUA−Cu₂O and Au−cys−Cu₂O showed clearly only the localized SPR (LSPR) effect, while Au−Cu₂O demonstrated the coexistence of the two effects, which was further confirmed by their LSPR enhancement factor.