The penetration of toner into porous substrates is studied using cross sections and topographical maps of the toner layer. Toner penetrates readily into the pores of the substrate and the penetration depth is larger than previously stated. As the toner enters the fixing stage, its temperature increases, which leads to a decrease in its viscosity. As a result, the toner penetrates and flows more easily into the pores of the substrate. The nip pressure pushes the toner layer against the substrate, thus promoting the penetration significantly. The toner layer also adapts to the irregularities of the paper surface. Toner does not seem to penetrate into 0.1 μm pores. This indicates that toner cannot penetrate into the pores of paper coatings. The surface energy of the substrate seems to increase toner penetration slightly by enhancing wetting, spreading, and penetration potential. Other substrate properties were not observed to significantly affect toner penetration. However, these effects of the substrate properties are almost totally overrun by the pressure in the nip.