The energetics of electron transfer in DNA has been estimated via differences of electron affinities (EAs) of nucleobases B in trimers of Watson–Crick pairs 5′-XBY-3′ calculated with the semi-empirical method AM1. The EA values of the nucleobases were found to decrease in the order . The destabilizing effect of the subsequent base Y is more pronounced than that of the preceding base X. We predict that the strongest electron traps are 5′-XCY-3′ and 5′-XTY-3′, where X, Y are pyrimidines C and T. These triads exhibit very similar EA values, and therefore, the corresponding anion radical states should be approximately in resonance, favoring efficient transport of excess electrons in DNA.