The one-electron reduction of Ru(bpy)(2)(AB)(2+), hereafter RuAB(2+), where bpy = 2,2＇-bipyridine and AB = 2,2＇:3＇,2 ":6 ",2＇"-quaterpyridine, in aqueous solution has been studied using pulse radiolysis. RuAB(2+) is reduced by e(aq)(-) (k = 6.2 x 10(10) M-1 s(-1) at ambient ionic strength (I) and k = 4.6 x 10(10) M-1 s(-1) at I = 0.1 M) and by CO2.- radical (k = 1 x 10(8) M-1 s(-1) at I = 0.1 M), generating RuAB(.+). The spectra of RuAB(.+) and its conjugated acid (RuABH(. 2+)) were obtained; the pK(a) of RuABH(. 2+) (8.0) has been determined. The rate constants for the protonation of RuAB(.+) (k(H) = 7.9 x 10(9) M-1 s(-1)) and the deprotonation of RuABH(. 2+) (k(-H) = 79 s(-1)) have been evaluated as have the rate constants for proton exchange (k(f) = 3.4 x 10(7) M-1 s(-1) and k(r) = 5.5 x 10(6) M-1 s(-1)) between the RuAB(.+)/RuABH(. 2+) and H2PO4-/HPO42- couples. The results from this work are compared with those obtained previously for 13 Ru(II) complexes that contain bpy, bpm (2,2＇-bipyrimidine), bpz (2,2＇-bipyrazine), and pypm (2-(2＇-pyridyl)pyrimidine) ligands. In addition, since RuAB(2+) can be viewed as a substituted Ru(bpy)(3)(2+) complex that contains a bpy residue, a comparison is also made of the reductions of RuAB(2+) and bpy.