The electrochemical spectral sensitization of a $SnO_2$ electrode with eosin B, a Xanthene dye, has been studied in acetonitrile. Measurements of the photocurrent have been carried out in the presence of supersensitizers such as thiourea, 1-allyl-2-thiourea, NaSCN, and NaI. The magnitude of the supersensitized photocurrent was greater than that of the sensitized photocurrent for all of the supersensitizers studied. However, the long time span of irradiation causes a significant decrease of the supersensitized photocurrent as well as the absorbance. These results, together with infrared spectra and fluorescence spectra, are taken into account to elucidate the mechanism of photoreaction between eosin B and supersensitizers in acetonitrile.