The inkjet printing of a polymeric solution into a porous substrate was studied, with the focus on phenomena occurring within the pore space during infiltration. Lines of aqueous polyacrylic acid (PAA) solution were printed onto the surface of porous, high-green-density ceramic powder beds. The PAA is a binder for the ceramic particles, allowing removal of the printed line structure ("primitive") and characterization of the extent of polymer penetration. Large differences in cross section of the retrieved printed structure were observed between ceramic systems and for different specific surface area powders. A mechanism for "filtration" of the polymer by adsorption onto the ceramic particle surfaces during infiltration was proposed. The adsorption of PAA onto Al2O3, SiO2, and TiO2 was characterized via adsorption isotherms, and the trend of primitive cross section with PAA adsorption was consistent with the filtration hypothesis, as was the variation with powder-specific surface area. These results can be generalized to other systems where a solution is inkjet printed onto a porous substrate (e.g., inks on plain paper, porous coated papers, etc.) Utilization of the adsorption effects may allow confinement of the solute molecules (e.g., colorant) to a small region near the substrate surface.