In this article, the deposition and characterization of amorphous MgO films grown on Si (001) using molecular beam epitaxy is reported. In order to ensure amorphous films, low substrate temperatures (200 degrees C) and high oxygen pressures were used (up to 5 x 10(-3) Pa). Both atomic and molecular oxygen species were used at different pressures. Films ranging in thickness from 3 nm till 30 nm were grown and characterized using structural and electrical methods. The dielectric constant deduced from this thickness series corresponds to 8.74 close to the bulk value of 9.8. The best equivalent oxide thickness value observed for this series grown under high oxygen pressure corresponds to 12 nm for a current density measured at V-FB - 1 V of 0.1 A/cm(2). The films grown with atomic oxygen display a lower capacitance and lower leakage compared to the films grown using molecular oxygen. This is due to the appearance of a thin SiO2 interface layer which is also responsible for the high defect density observed in the capacitance voltage measurements. Upon annealing both types of films in forming gas for 15 min at 450 degrees C, the capacitance decreases further while the leakage increases. This change is assigned to the appearance of the lower bandgap (6.6 eV), lower dielectric constant (6.6) phase of silicate, MgSiO3. Furthermore a V-FB shift of -0.6 V is observed and is related to the appearance of oxygen vacancies as Mg diffuses into the SiO2 interface layer. 2011 Published by Elsevier B.V.