Abstract
The momentum distribution of positronium (Ps) in the free space between silica aerogel grains filled with gaseous O2 has been measured as a function of O2 pressure. It reveals that para-Ps with kinetic energy >or approximately=1 eV is efficiently slowed down. This is attributed to successive triplet to or from singlet conversions of the Ps through inelastic collisions with O2. The cross section for this process is estimated to be at least two orders of magnitude larger than that for the conversion through elastic collisions.