Photoemission current at a mercury electrode has been characterized electrochemically for aqueous solutions of methyl viologen, the disodium salt of anthraquinone-1,5-disulfonic acid, hydroquinone, p-benzoquinone, and chlorophyllin, and in DMF solutions of N2O and anthraquinone. The scavenging species could be determined by comparing photoemission current-voltage curves with polarographic and cyclic voltammetric data.