Abstract The thermodynamic equilibrium of the association of macromolecular subunits to larger particles is described by the concept of successive binding to existing aggregates. The binding is assumed to be cooperative, i.e., the dimerization constant is much smaller than the binding constants of monomers to aggregates. Two theoretical models are discussed in detail. In the first model the binding constants are assumed to be independent of the particle size, whereas in the second model the binding constants decrease with increasing particle size. Both models predict the existence of a critical monomer concentration which marks the onset of aggregation in the case of high cooperativity. The number and weight averages of the particle size in both models are discussed. A maximum in the size distribution of the aggregates is obtained only in the second model.