Abstract We have studied the (1 1 0) surfaces of Si and Ge using first principles total energy calculations. Although we did not perform a complete calculation of the (16×2) reconstruction of Si(1 1 0), we have studied smaller systems that separately contain the basic building blocks of the reconstruction. They are important components of several models of the atomic structure of the Si and Ge(1 1 0) surfaces and they can also give us important physical information. In particular, we have considered the bond conserving rotation model, similar to the relaxation pattern observed in (1 1 0) surfaces of zincblend compounds and two different adatom structures. It is found that by simply relaxing, the Si(1 1 0) surface gain 0.67 eV/[(1×1) cell] with respect to the ideally bulk terminated surface. This value is increased if the orientation of the buckling changes from row to row. However, we found that an adatom structure is even more favorable energetically. Different, from the case of (1 1 1) surfaces, there is no adatom rest-atom mechanism. Instead, each adatom bonds with four first layer atoms, leaving no rest atoms.