Using the semi-empirical scheme NDDO-G, we estimated energies for hole transfer in DNA duplexes on the basis of calculated ionization potentials (IP) of nucleobases in triplets of regular structure. All possible triad 5'-XBY-3' duplexes (X,B,Y = A,G,C,T) were considered. We find that the stabilization of B+ is considerably influenced by the subsequent base Y while the effect of the preceding base X is rather small. We test the application of triplet models by comparison with a decamer duplex. The present semi-empirical results have also been compared with data from ab initio calculations and experiment.