The principle of the equivalence of gravitational and inertial mass is one of the cornerstones of general relativity. Considerable efforts have been made and are still being made to verify its validity. A quantum-mechanical formulation of gravity allows for non-Newtonian contributions to the force which might lead to a difference in the gravitational force on matter and antimatter. While it is widely expected that the gravitational interaction of matter and of antimatter should be identical, this assertion has never been tested experimentally. With the production of large amounts of cold antihydrogen at the CERN Antiproton Decelerator, such a test with neutral antimatter atoms has now become feasible. For this purpose, we have proposed to set up the AEGIS experiment at 0168-583X/$ -see front matter Ó 2007 Published by Elsevier B.V.

The comparison of different models (the Ore, spur and blob models) of positronium (Ps) formation is presented. Because in molecular media Ps is formed in the terminal positron blob and not in an ordinary spur, the application of the blob model seems to be the most adequate. We extend this model for consideration of the Ps formation in the presence of external electric field ($<100$ kV/cm). In the simplified limiting case, this approach provides a formula similar to the Onsager one for the geminate recombination probability. The influence of ion-electron recombination and other intrablob processes on Ps formation is taken into account. The role of quasifree positronium in Ps formation process is discussed

The interaction of positronium with molecular oxygen dissolved in liquids is experimentally investigated. A computer software has been developed for fitting the positron annihilation lifetime spectra using parameters with clear physical meaning.