Size distribution functions of diamond particles formed in the early stages of chemical vapour deposition (CVD) can be used to distinguish between seeding and heterogeneous nucleation on the basis of their shape and of the d(max)/d(min) ratio, d(max) and d(min), being the maximum and the minimum diameters, respectively. A monomodal size distribution function with a d(max)/d(min) ratio much greater than 1.2-1.3 indicates diamond formation to occur via heterogeneous nucleation. In this case the nucleation kinetics can be calculated once the growth law of the crystallites has been established. The nucleation kinetics at copper substrates have been derived from distribution functions and described by a new kinetic model which includes the generation of nucleation sites.