SiO2 films were grown in dry oxygen on Si (100) substrates at various temperatures, then annealed for various times and temperatures. The density of the oxides was found to depend on the growth temperature, and on the anneal time and temperature. The amount of positive charge trapped in these oxides during vacuum ultraviolet hole injection was found to be inversely related to the oxide density, regardless of how the oxide was prepared. Comparison of the density of trapped holes with the density of E' centers detected by electron spin resonance indicates that E' centers cannot account for most of the trapped positive charge. A model to explain these observations is proposed.