Lateral interactions between adatoms on a solid surface play a key role in determining whether adsorption takes place dissociatively or non-dissociatively, as revealed by single crystal adsorption calorimetry. With NO on Ni{100}, adsorption occurs dissociatively at low coverage, while at higher coverage strong interatomic repulsions reverse the relative stability and the molecularly adsorbed state is then more stable than the dissociated state. Essentially, above a critical adatom coverage the adsorption heat for further dissociative adsorption is lower than that for non-dissociative adsorption, due to strong repulsive interactions between adatoms on the surface. By changing the oxygen adatom precoverage it is possible to control the relative amounts of dissociated and molecularly adsorbed NO. This result has important general consequences for the control of catalytic reactions.