Double proton transfer (DPT) in guanine-cytosine (GC) base pairs and adenine-thymine (AT) base pairs produces tautomeric forms, denoted G*C* and A*T*. To examine DPT, (i) intrinsic reaction coordinates for DPT, (ii) probabilities of change from GC to G*C* and from AT to A*T*, and (iii) infrared absorption intensities of GC and G*C* were investigated on the basis of density functional theory and Eyring's chemical kinetics. The probabilities of change from GC to G*C* were 3 x 10(-8), and those from AT to A*T* were 2 x 10(-40). These values are consistent with the rate of mutation previously reported by Drake et al. (Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 7160-7164). G*C* exhibited two vibrational modes around 3000 cm(-1), whereas GC exhibited no vibrational modes around the same frequency. The infrared intensity calculated for G*C* showed that the strong absorption obtained around 3000 cm(-1) was caused by one of the two modes.