The electron transfer reaction between triplet anthraquinone-2-sulfonate with poly-guanylic acid (5) (poly[G]) in acetonitrile-H2O has been investigated by 248 nm laser flash photolysis. The transient absorption spectra of radical cation of poly[G] and radical anion of anthraqunione-2-sulfonate (AQS) arising from electron transfer reaction has been observed simultaneously for the first time. The formation processes of radical cation of poly[G] and radical anion of anthraquinone-2-sulfonate as well as the decay processes of triplet anthraquinone-2-sulfonate have also been observed, the apparent rate constants for the formation and decay of transient species have been determined. The free energy changes in the process of the electron transfer was also calculated. The electron transfer reaction between triplet anthraquinone-2-sulfonate with poly-guanylic acid (5) (poly [G]) in acetonitrile-H2O has been investigated by 248 nm laser flash photolysis. The transient absorption spectra of radical cation of poly [G ] and radical anion of anthraqunione-2-sulfonate (AQS) arising from electron transfer reaction has been observed simultaneously for the first time. The formation processes of radical cation of poly [G] and radical anion of anthraquinone-2-sulfonate as well as the decay processes of triplet anthraquinone-2-sulfonate have also been observed, the apparent rate constants for the formation and decay of transient species have been determined. The free energy changes in the process of the electron transfer was also calculated.