Electronic energy relaxation in the adenine homopolymers poly(A) and poly(dA), as well as in the adenine mononucleotide, was studied by the femtosecond transient absorption technique and by steady-state absorption and emission spectroscopies in aqueous solution. The excited-state lifetime of the adenine mononucleotide was determined to be 370 40 A at room temperature. Strikingly, the singlet excited states formed in adenine homopolymers decay on time scales ranging from femtoseconds to nanoseconds, and three decay components are required to adequately describe their transient signals. Increasing the temperature decreases the amplitude of the two slowest decay components, indicating that the long-lived excited states are formed in base-stacked regions of the polymers. At room temperature and neutral pH, the two slowest decay components have significantly larger amplitudes in poly(dA) than in poly(A). No evidence is obtained for excited-state quenching of the adenine chromophore by the nonstandard base pairing that occurs in the double-stranded form of poly(A) at acidic pH. Instead, double helix formation slightly increases the yield of long-lived excitations. This work demonstrates that electronic energy relaxation depends sensitively on the secondary structure of the adenine homopolymers.