Brexpiprazole which marketed as Rexulti® is a second-generation atypical antipsychotic. A seemingly random and uncontrollable “ghost peak” that vary from below the detection limit to ∼0.2% was observed occasionally during the development of HPLC method for brexpiprazole drug substance. The “ghost peak” was finally characterized as a syn head-to-tail dimer of brexpiprazole by UV, IR, LC–MS/TOF, 1D and 2D NMR. It was formed rapidly through photocatalytic [2 + 2] pericyclic reaction and can reach approximately 1.7% in five minutes once the analytes were exposed to sunlight at noon. By contrast, the occurrence of this “ghost peak” in brexpiprazole can be suppressed when the solutions were prepared and stored away from sunlight.