  An analysis of the kinetics of H2 formation on interstellar dust grains is
presented using rate equations. It is shown that semi-empirical expressions
that appeared in the literature represent two different physical regimes. In
particular, it is shown that the expression given by Hollenbach, Werner and
Salpeter [ApJ, 163, 165 (1971)] applies when high flux, or high mobility, of H
atoms on the surface of a grain, makes it very unlikely that H atoms evaporate
before they meet each other and recombine. The expression of Pirronello et al.\
[ApJ, 483, L131 (1997)] -- deduced on the basis of accurate measurements on
realistic dust analogue -- applies to the opposite regime (low coverage and low
mobility). The implications of this analysis for the understanding of the
processes dominating in the Interstellar Medium are discussed.
