The grain size in polycrystalline silicon layers is subject to various external conditions among which the initial nucleation of silicon on the substrate may play a decisive role. This article describes experiments with silicon on and substrates in the system for temperatures between 925° and 1200°C. The saturation cluster densities are determined as a function of gas phase composition and temperature and could be varied between 104–1011 cm−2. The experimental results are compared with predictions of existing nucleation theories using analysis of the monomer silicon adatom concentration. In this way the nucleation can be described satisfactorily and approximate values for the size of the critical cluster can be derived.