Cluster-cluster collisions and cluster dissociation into two smaller clusters have an effect on steady-state size distributions and nucleation rates. Clustering also affects nucleation rates by influencing the saturation vapour pressure, which is often ignored in nucleation studies. Both the kinetic and the vapour pressure effect of clustering on nucleation in one and two component systems are investigated. For water, the effect of cluster-cluster kinetics is in the order of 10%, and the effect via vapour pressure in the order of 100% in nucleation rate. The approximate method [Shugard et al., J. Chem. Phys. 75, 5298 (1974)] to take the kinetic effect of clustering into account works well for water. In acetic acid vapour the dimer concentrations are an order of magnitude higher than monomer concentrations. Even in this extreme case the approximate way to describe the kinetic effect of clustering gives reasonable estimates of the comet rates. In sulphuric acid-water mixture the kinetic effect is from one to four orders of magnitude in nucleation rate, depending on whether the clustering is accounted for in the saturation vapour pressure or not. The effect of clustering via vapour pressure is 5-10 orders of magnitude, depending on the kinetic model used. The approximate way to describe cluster kinetics works only in some of the cases. (C) 1997 American Institute of Physics.