The details of muonium formation via recombination of the thermalized
muon and electron are discussed on the basis of mu SR experiments in c
ondensed neon, which are interpreted in the framework of the isolated
mu(+) - e(-) pair model. The comparison of the results in different su
bstances is made, In this context, the limits of the model, as well as
possible extra factors, which can affect muonium formation, are discu
ssed.