Honeycomb sandwich structures made of fiber-reinforced polymer facesheets, film adhesive, and core can be produced using co-cure, during which the facesheets and adhesive are processed simultaneously. Frequently, honeycomb skins (facesheets and adhesive) must be impermeable during service to protect the core from environmental degradation or maintain internal pressure. However, skins can be rendered permeable by process-induced defects such as porosity. This paper describes an investigation of relationships between process conditions during co-cure and skin permeability. Half-sandwich structures composed of a four-ply prepreg laminate, a layer of film adhesive, and a honeycomb core were fabricated using a custom designed lab-scale fixture and controlled thermal and pressure conditions. Autoclave, bag, and core pressures were varied to evaluate hypotheses about mechanisms leading to permeability. Parts were cured with vacuum drawn in the bag and core (resulting in porosity), with super-ambient bag and core pressures (leading to minimal porosity), or with several constant pressure gradients between bag and core (leading to possible continuous flow between these regions). The resulting skins were tested for permeability within a second custom-designed fixture that allowed accurate, repeatable gas flow rate measurements across the skins. Gas flow pathways were also located using a water layer. The micro-structure of the skins was analyzed using light microscopy of polished sections to assess the presence, amount, location, and distribution of porosity and other defects. Results showed that porosity is related to permeability, but that gas flow through skins is possible even without high void contents. Pressure differentials led to complex variations in permeability, showing that variations in gas flow rate between cured skins is at least partly stochastic. Overall, the study contributes to a fundamental understanding of processing and manufacturing science for composites, and can provide useful guidelines for improving manufacturing processes and ensuring part quality.