We apply molecular dynamics (MD) simulations to the study of the association of nonpolar spheres of effective radii between 1.6 and 6.1 Angstrom dissolved in water. The constrained MD method is used to calculate the potential of mean force (PMF) of the interaction between spheres. The depth of the potential of mean force increases with increasing radius of the nonpolar sphere. Our results suggest that the PMF is largely governed by size or entropic effects, and that energetic effects associated with the breaking or distortion of hydrogen bonds are of minor importance.