To assess the importance of pair and three-body interactions between adatoms we have made field-ion-microscopic observations comparing the behavior of two with three and more rhenium atoms on the W(110) plane. For two adatoms, close dimers are not stable, and measurements of the radial distribution function confirm that pair interactions are repulsive out to 10. Despite that, Re trimers form from three adatoms. Quantitative studies of trimer dissociation lead to a binding energy due to trio interactions of 0.25 eV. © 1984 The American Physical Society.