The diffusion coefficient of clusters 𝒟 formed by cluster–cluster aggregation is computed according to the Kirkwood–Riseman Theory. In three dimensions one finds 𝒟 proportional to sγ where s is the number of particles in the cluster and γ=−0.544±0.014. This relationship is employed to simulate the time evolution of the cluster size distribution Ns(t) which is found to exhibit simple scaling behavior Ns(t) ∼s−2g(s/tz) with z∼1.1.