In a neutral aqueous solution of acridine-1,8-dione (AD) the e(aq)(-) was observed to react with a bimolecular rate constant of 1.5 x 10(10) dm(3) mol(-1) s(-1) and form transient optical absorption bands with lambda(max) = 305, 475 nm, a shoulder at 680 nm, and increasing absorption with lambda > 800 nm. The entire spectrum from lambda > 450 nm decayed by first-order kinetics with k = 1.5 x 10(5) s(-1), whereas the absorption at 315 nm showed growth with k = 1.6 x 10(5) s(-1). It decayed by second-order kinetics with 2k/epsilon l = 1 x 10(4) s(-1). The transient optical absorption bands at 305, 475, 680, and >800 nm are assigned to the electron adduct, AD(.-), which on protonation forms H adduct (ADH(.)) with lambda(max) = 315 nm and a pK value of 4.7. In basic solutions (pH = 13), AD(.-) does not show slow transformation to ADH(.) while in acidic solutions (pH = 3), only ADH(.) was observed (lambda(max) = 315 nm). ADH(.) in highly acidic solutions (pH < 3) protonates to form (ADH(2))(.+) (lambda(max) = 315, 560 nm) with a pK value of 2.6. AD/AD(.-) established an equilibrium with MV2+/MV.+ and the reduction potential for AD/AD(.-) couple was determined to be -0.586 +/- 0.03 V. alpha-Hydroxyalkyl radicals (CH3)(2)(COH)-O-., (CH3CHOH)-H-., and (CH2OH)-C-.) were able to reduce AD with bimolecular rate constant values of 3.2 x 10(8), 2.3 x 10(8), and 1.2 x 10(8) dm(3) mol(-1) s(-1), respectively. The radical anion AD(.-) was able to transfer electron to p-nitroacetophenone with a bimolecular rate constant of 6 x 10(9) dm(3) mol(-1) s(-1).