Kinetics of a photoinduced electron-transfer reaction between phenothiazine (PTZ) and Ru(bpy)(2)(4-m-4'-pa-bpy)(2+) [bpy = 2,2'-bipyridine and 4-m-4'-pa-bpy = 4-methyl-4'-carbonylpropargylamine] covalently bound to a DNA duplex is investigated by transient absorption spectroscopy. The electron donor, PTZ, is attached to the 5' terminal of one oligodeoxynucleotide strand, and the chromophore, Ru(bpy)2(4-m-4'-pa-bpy)2+, is covalently linked to a uridine of the complementary strand. Electron transfer between the donor, PTZ, and photoexcited acceptor, *Ru(bpy)(2)(4-m-4'-pa-bpy)(2+), occurs, and the product of the forward electron-transfer reaction, PTZ(+.), is observed. The rate of the back electron-transfer reaction (k(b) = 3.94 x 10(6) s(-1); Delta G similar to 2.0 eV) is determined by monitoring the decay of the electron-transfer intermediate, PTZ(+.), by transient absorption spectroscopy.