Lifetimes of grains in H I clouds against destruction in supernova blast waves are only ~ 3 × 108 yr (e.g, Draine & Salpeter 1979, Jones et al 1994). How then can we understand the very small gas-phase abundances routinely found for elements like Si? The kinetics of depletion have been studied by Weingartner & Draine (1999), who show that the population of very small grains is capable of accreting metal ions rapidly enough to achieve the required depletions, provided that interstellar material is rapidly cycled between dense clouds and diffuse clouds.