Experimental results, obtained using a reflectivity method, for the probability of physical adsorption of ethane on the Ir( 110)–(1 X2) surface are presented. We analyze these results using Monte Carlo simulations and show that physical adsorption can occur either directly or through a precursor state in which an ethane molecule is trapped in a second layer above a first layer of physically adsorbed ethane. From the Monte Carlo simulations, we are able to establish that the energy barrier for desorption of an ethane molecule from the precursor state is ~4.5 kcal/mol. We also find that the energy barrier for diffusion of an ethane molecule on top of a monolayer of physically adsorbed ethane on the Ir(110) surface is ~ 3.7 kcal/mol. © 1990, American Vacuum Society. All rights reserved.