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Porous silica has been used as support for catalysts used to convert renewable and fossil resources into useful energy sources. Understanding the pore structure of these materials is crucial for the design and synthesis of supported metal catalysts. Positron annihilation lifetime spectroscopy (PALS) can be used to characterize the nanoscale pore structure of these materials. Silica powders with two different porous structures were subjected to compressive pressure deformation using a hydraulic press. SEM images at 100K magnification show that the more-porous silica has larger granules compared to low-porous silica. A decrease in granular size due to deformation is clearly observed for more-porous silica and almost unchanged for low-porous silica. The positron third lifetime component in the more-porous silica showed 6.6 ns lifetime (pore size of ~ 11 Å) with 3.4% intensity, while these values after 20 kPSI pressure deformation were 2.9 ns (pore size of ~ 6.8 Å) with 12.4% intensity. This shows that the larger-pores collapse to form 3-4 times more smaller-pores and consistent with the almost same values observed for the fractional free volume in the uncompressed and deformed samples. The effect of deformation on the low-porous silica is negligible at the applied pressure. The third lifetime for this sample is 1.8 ns (pore size of ~ 5.2 Å) with intensity of 4.3% for both as-made and deformed samples.