The electron effective mass for direct tunneling (EETM) in silicon dioxide NO2) was determined by many authors using a high bias field for obtaining direct measurable direct tunneling current densities (TCDs), leaving some ambiguity to the electron kinetic energy at/ within the SiO2 barrier. A determination of the EETM by exploiting a long term intrinsic charge-up process of vacant defects adjacent to the SiO2 barrier by capacitance voltage (CV) measurements is reported. The EETM is obtained by using the fixed charge sheet densities for iterating the TCD as a function of time. The zero bias field renders the average electron kinetic energy to converge on the thermal energy, leaving less ambiguity to the EETM. For an SiO2 layer of d(SiO2) = 8.2 nm, an EETM of m(eff) = (0.3 +/- 0.03)m(0) is obtained. (c) 2007 Elsevier Ltd. All rights reserved.