We investigate the porosity of a series of xerogels prepared from arylene-bridged silsesquioxane xerogels as a function of organic bridging group, condensation catalyst and post-synthesis plasma treatment to remove the organic functionalities. We conclude that porosity is controlled by polymer-solvent phase separation in the solution with no evidence of organic-inorganic phase separation. As the polymer grows and crosslinks, it becomes increasingly incompatible with the solvent and eventually microphase separates. The domain structure is controlled by a balance of network elasticity and non-bonding polymer-solvent interactions. The bridging organic groups serve to ameliorate polymer-solvent incompatibility. As a result, when the polymer does eventually phase separate, the rather tightly crosslinked network limits domain size to tens of angstroms, substantially smaller than that observed in xerogels obtained from purely inorganic precursors where incompatibility drives phase separation earlier in the gelation sequence.