Quantitative observations in the field ion microscope have been made of rhenium dimers and trimers on the (110) plane of tungsten. For Re pairs, the radial distribution function under equilibrium conditions has been measured at 400 K on a plane for which the distance scale was first established by repeated observation of a single adatom diffusing over the surface. From these measurements it appears that at T≈400 K there is little likelihood of two rhenium adatoms being closer than 8 Å to each other; interactions are repulsive out to an interatomic separation of ≈10 Å, in agreement with previous studies. Despite repulsive pair interactions, three rhenium atoms deposited on W(110) readily form a trimer at T≈400-450 K. Linear as well as triangular trimers have been identified, with the latter proving slightly more stable. The binding of Re atoms in these trimers has been studied in two ways. From measurements of the lifetime for dissociation at 420 K, we infer an activation energy to trimer dissociation of 1.29 eV. This leads to a binding energy of 0.25 eV for Re in a trimer after allowing for a barrier to diffusion of 1.04 eV. The equilibrium between adatoms and trimers at T = 435 K has also been studied. Quantitative examination of the thermodynamics yields a standard free energy difference of 0.27 eV between adatoms and triangular trimers, in good agreement with the results of the kinetic studies. In rhenium trimers on W(110) cohesion appears due entirely to trio interactions. Although this situation may prove to be unique, trio effects should play a substantial role in other adlayers as well. © 1984 American Institute of Physics.