Atomic scale properties of thin porous silicon (PSi) layers, characterized by the formation of positronium, are investigated using positron annihilation lifetime spectroscopy in the temperature range 20-300 K under 10(-7) Torr vacuum. The longest orthopositronium as well as the shortest parapositronium components are found to have quite low intensities in the thin layer at room temperature. It is also found that at temperatures <= 240 K, these two components do not show up in the spectrum. The reason for this absence of the longest lifetime component is suggested. (c) 2006 Elsevier B.V. All rights reserved.