AbstractThe contact‐complex formation reaction between solvent molecules in square‐well fluids is studied for the two‐ and three‐dimensional system. The effect of the third body at the second‐virial limit is expressed by the following three factors: (1) the excluded volume effect, (2) the desolvation effect, and (3) the „glue effect”︁, where this effect means that the third body behaves like a glue binding a pair of molecules. The density derivative of the equilibrium constant of the contact‐complex formation, at the second‐virial limit, goes through a minimum with increasing temperature in both the two‐ and three‐dimensional fluids, whereas it increases monotonically in the one‐dimensional fluid. However, a Monte Carlo simulation shows that the difference due to the dimensionality of the system is rapidly smeared out with increasing density. The equilibrium constants of the contact‐complex formation in the two‐ and three‐dimensional square‐well fluids have qualitatively the same temperature and density dependence as in the one‐dimensional fluid at densities about 0.5 times higher than their critical densities