A mathematical model of the formation of primary grown-in microdefects on the basis of dissociation diffusion is presented. Cases of “vacancy–oxygen” (V + O) and “carbon–interstitial” (C + I) interaction near the crystallization front are considered for dislocation-free Si single crystals grown by the floating-zone and Czochralski methods. The approximate analytical expressions obtained by setting 1D and 2D temperature fields in a crystal are in good agreement with the heterogeneous mechanism of formation of grown-in microdefects.