We report the synthesis of colloidal Ni2+-doped SnO2 (Ni2+:SnO2) nanocrystals and their characterization by electronic absorption, magnetic circular dichroism, X-ray absorption, magnetic susceptibility, scanning electron microscopy, and X-ray diffraction measurements. The Ni2+ clopants are found to occupy pseudooctahedral Sn4+ cation sites of rutile SnO2 without local charge compensation. The paramagnetic nanocrystals exhibit robust high-Curie-temperature (Tc) ferromagnetism (M-s(300 K) = 0.8 mu B/Ni2+, Tc >> 300 K) when spin-coated into films, attributed to the formation of interfacial fusion defects. Facile reversibility of the paramagnetic-ferromagnetic phase transition is also observed. This magnetic phase transition is studied as a function of temperature, time, and atmospheric composition, from which the barrier to ferromagnetic activation (Ea) is estimated to be 1200 cm(-1). This energy is associated with ligand mobility on the surfaces of the Ni2+:SnO2 nanocrystals. The phase transition is reversed under air but not under N-2, from which the microscopic identity of the activating defect is proposed to be interfacial oxygen vacancies.