We propose a simple method to optimize silane-to-oxygen flow ratio in low pressure, high-density plasma enhanced chemical vapor deposition of SiO2 from silane/oxygen mixtures. It is well known that the increase of SiH4/O2 flow ratio R results in degradation of oxide quality. We have found that the critical flow ratio can be estimated from the analysis of the difference between gas pressures with and without plasma. The evolution of optical properties of the films shows that the critical value obtained from pressure measurements gives a good indication of the limits for the deposition of optical quality SiO2. © 2004 American Vacuum Society.