We study non-doped dislocation-free monocrystals of float-zone silicon using transmission electronic microscopy, optical microscopy and x-ray topography. The crystals were obtained with various growth rates (1–9 mm min−1) and were subjected to various kinds of thermal processing. We experimentally determine the temperatures at which microdefects of various types form, and we establish the mechanism of transformation of interstitial microdefects. On the basis of data in the literature and new results obtained by authors, we establish that the formation of microdefects in silicon occurs on two independent mechanisms: vacancy and interstitial. As a result of both these mechanisms, D-microdefects will be formed as interstitials agglomerate. We suggest that the critical parameter V/G = Ccrit describes the conditions of emerging (vanishing) vacancy microdefects. On the basis of these results, we suggest a physical model of the formation of microdefects in dislocation-free monocrystals of float-zone silicon, and we discuss other known models.