The formation of Si quantum dots on SiO2 by chemical vapour deposition of SiH4 is investigated by energy filtered transmission electron microscopy. It is demonstrated that this technique allows to measure size distributions down to dimensions of about 1nm. This capability allows to put in evidence some important microscopic features of the nucleation process, whose consideration is fundamental to control the Si dot size. These aspects are shown and discussed. (C) 2002 Elsevier Science B.V. All rights reserved.