Some vesicles in certain subaerial flows of basalt and basaltic andesite are partially filled with dark, partly-glassy segregation material. The segregation material has about three times the concentration of K₂O, TiO₂, and P₂O₅ as the associated host rock. Therefore, the segregation materials are inferred to be frozen residual liquids that migrated into the vesicles. Several possible processes for filling the vesicles are examined within the context of the cooling time implied by heat conduction. The preferred process is one in which the residual liquid migrates through a porous and permeable, but rigid, network of interlocking crystals in response to a pressure gradient generated by vapor saturated crystallization. As crystallization proceeds H₂O is concentrated in the residual liquid and gas. This produces a higher pressure in the gas-poor matrix than in the vesicles and forces liquid to migrate into the vesicles. Textural features suggestive of or consistent with the process include: (1) vesiculation ...