Investigations into the physical characteristics of deep firn near the lock-in zone through pore close-off are needed to improve understanding of ice core records of past atmospheric concentrations. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age-gas age difference. The purpose of this study is to examine the nature of pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from WAIS Divide in West Antarctica and Megadunes in East Antarctica. We found that the open pore structure plays a more important role than density in predicting gas transport properties, through the porous firn matrix. For both WAIS Divide and Megadunes, fine grained layers experience close-off shallower in the firn column than do coarse grained layers, regardless of which grain sized layer is the more dense layer at depth. Pore close-off occurs at an open porosity that is accumulation rate dependent. Low accumulation sites, with coarse grains, close-off at lower open porosities (〈 10%) than the open porosity (〉 10%) of high accumulation sites with finer grains. The depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and pore structure, rather than the density variability of the layers.