A kinetic model is presented which explains the nonstatistical cluster distributions found in a supersonic expansion of binary mixtures of weakly interacting organic molecules. It is shown that clustering in a supersonic expansion leads to a fractionation of the clusters, i.e., to an enrichment of the most strongly bound component. In our model we consider in the step-by-step growing process of the cluster three important events. In a first step the cluster grows by the addition of a monomer and forms an activated complex. This complex can be stabilized by a collision with an atom of the monatomic carrier gas, or it can be stabilized by a branching process in which it loses the same type of monomer or by a substitution process in which the other type of molecule is leaving. As follows from the RRK equations, for the addition of the strongly bound component the substitution reaction rate is much faster than for the addition of the more weakly bound component. This gives rise to the nonstatistical enrichment toward the more strongly bound species as observed. For comparison with our experimental results, it is important to include the physics of the supersonic expansion process in our model calculation, because it is essential that the clustering process freezes in at an early stage. We show that the experimental results can be accounted for with our kinetic model with only two parameters : the stabilization efficiency of the monatomic carrier gas and the average temperature of the clusters.