Mg clusters embedded in Ar droplets of different size (about 100 and 3500 Ar atoms, respectively) were formed in a pick-up cell and subsequently analysed by mass spectroscopy after electron beam ionisation. Comparison with previously published data for He droplets in Phys. Rev. Lett. 86 (2001) 4807 leads to the conclusion that the larger clusters undergo substantial fragmentation upon ionisation and that the resulting intensity distributions of the ions do not reflect the relative stabilities of the neutral species, which had been considered as evidence for electron delocalisation in clusters counting more than about 20 Mg atoms. (C) 2003 Elsevier B.V. All rights reserved.