The deposition of conformal coatings into porous silicon layers was successfully demonstrated. Tin oxide films were formed from SnCl4 and H2O precursors by atomic layer epitaxy. The influence of the porous substrate structure on the deposition parameters was analyzed from the viewpoint of formation mechanism growth rate, and layer composition. The SnOx covered porous substrates were characterized by means of Rutherford backscattering, secondary ion mass spectrometry : cross-sectional transmission electron microscopy, and ellipsometry. The mesoporous structure of the Si substrate uniquely determines the gas-phase diffusion and physisorption of the precursors. The processing parameters favoring chemisorption are more critical for porous silicon than those for a flat surface. Even a small decrease in the deposition temperature results in a considerable increase in the growth rate through gas-phase reactions, and the process becomes chemical vapor deposition-like. Conformal step coverage was obtained on extremely high (140:1) aspect ratio pores if the deposition conditions were chosen such that chemisorption was the growth rate determining step in the process.