Lightly-doped silicon (Si) samples of n-type conductivity have been irradiated with 2.0 MeV [Formula: see text] ions at a temperature of 30 K and characterized in situ by deep level transient spectroscopy (DLTS) measurements using an on-line setup. Migration of the Si mono-vacancy in its double negative charge state (V 2-) starts to occur at temperatures above ∼70 K and is monitored via trapping of V 2- by interstitial oxygen impurity atoms ([Formula: see text]), leading to the growth of the prominent vacancy-oxygen ([Formula: see text]) center. The [Formula: see text] center gives rise to an acceptor level located at ∼0.17 eV below the conduction band edge (E c ) and is readily detected by DLTS measurements. Post-irradiation isothermal anneals at temperatures in the range of 70 to 90 K reveal first-order kinetics for the reaction [Formula: see text] in both Czochralski-grown and Float-zone samples subjected to low fluences of [Formula: see text] ions, i.e. the irradiation-induced V concentration is dilute ([Formula: see text]1013 cm-3). On the basis of these kinetics data and the content of [Formula: see text], the diffusivity of V 2- can be determined quantitatively and is found to exhibit an activation energy for migration of ∼0.18 eV with a pre-exponential factor of ∼[Formula: see text] cm2 s-1. The latter value evidences a simple jump process without any entropy effects for the motion of V 2-. No deep level in the bandgap to be associated with V 2- is observed but the results suggest that the level is situated deeper than ∼0.19 eV below E c , corroborating results reported previously in the literature.