We present a simple one-dimensional model with molecular interactions favoring the formation of clusters with a defined optimal size. Increasing the density, at low temperature, the system goes from a nearly ideal gas of independent molecules to a system with most of the molecules in optimal clusters, in a way that resembles the formation of micelles in a dilution of amphiphilic molecules, at the critical micellar concentration. Our model is simple enough to have an exact solution, but it contains some basic features of more realistic descriptions of amphiphilic systems: molecular excluded volume and molecular attractions which are saturated at the optimal cluster. The comparison between the exact results and the mean field density functional approximation suggests new approaches to study the more complex and realistic models of micelle formation; in particular it addresses the long-standing controversy surrounding the separation of internal degrees of freedom in the formulation of cluster association phenomena. (C) 1997 American Institute of Physics. [S0021-9606(97)51747-8].