The relation between the concentration of intrinsic paramagnetic states (theE′ centers) of silica (silica glass) produced by irradiation and the concentration of OH−ions in the silica was investigated by introducing OH−ions into silica, initially free of these ions, in a controlled manner. Optical absorption and electron spin resonance measurements were used to establish the relation. The results of the investigation showed that in synthetic silicas the concentration ofE′ centers varied linearly with the concentration of hydroxyl units, the sum of the concentrations of hydroxyl units andE′ centers being constant. A ``vacancy'' model for the silica structure is proposed. In terms of this model the estimated saturation concentration ofE′ centers is of the same order as the concentration of ``vacancies,'' ∼1020cm−3. The saturation concentration of hydroxyl units is also ∼1020cm−3. Silica made by fusing natural quartz crystals differs from the synthetic silica with respect to the optical bands and the electron spin resonance spectra produced by irradiation. The effects of introducing hydroxyl units into natural quartz silica initially free of hydroxyl units is also different. These differences between natural quartz silica and synthetic silica are related to the models which have been suggested for theE′ centers. The models which are suggested are in agreement with the results of other experiments.