  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.
