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 AMI. The EA values of the nucleobases were found to decrease in the order cytosine (C) approximate to thymine (T) much greater than adenine (A) > guanine (G). 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.