Positron lifetime spectroscopy and two‐dimensional angular correlation of annihilation radiation have been used to investigate grown‐in vacancy structures in synthetic crystalline &agr;‐SiO2, synthetic fused quartz, and in a 60‐&mgr;m‐thick chemical‐vapor‐deposited amorphous SiO2film. For &agr;‐SiO2a ∼300 ps lifetime component suggests trapping by either silicon monovacancies or by oxygen divacancies (or both). The vacancies are neutral and present at a concentration level of 1017/cm3. The positron bulk lifetime for &agr;‐SiO2is estimated to be ∼238 ps in good agreement with semiempirical predictions. In the fused quartz significant positronium formation is found (80%) and the remaining positrons annihilate in voids yielding a lifetime of ∼500 ps. The amorphous SiO2film contains a mixture of small vacancy clusters and voids and ∼30% of the positrons form positronium. Heat treatment above 950 °C results in a substantial reduction in defect concentration, but up to 1100 °C a small vacancy cluster contribution persists. The positron data indicate that positronium formation in the fused quartz and in the amorphous film takes place in the voids.