A time‐of‐flight mass spectroscopy technique was developed that enables continuous monitoring of ion mass distribution in the output of a typical ionized cluster beam (ICB) source with minimum modification of the setup and negligible disturbance of film growth. The low mass resolution of this spectrometer is sufficient for cluster size determination and is counterbalanced by a number of advantages promising a more general applicability of the technique. This spectrometer has been used to study germanium cluster formation in a commercially available ICB source. No clusters larger than a few atoms were observed during several deposition runs, under conditions relevant to practical applications. It is difficult to reconcile these findings with the original hypothesis of homogeneous nucleation and growth of large clusters by free jet expansion of metal vapor. Since the precise temperature gradient in the crucible could not be controlled in this type of source, it cannot be excluded that large clusters may be formed under very particular conditions. If this is the case, vapor condensation on the nozzle’s walls must play the key role in the formation of clusters, of germanium at least, making ICB less attractive as a semiconductor thin film growth technique.