The kinetics of methane trapping on ethylidyne-covered Pt(lll) were investigated using supersonic molecular beam techniques at surface temperatures between the temperature at which methane desorbs from the monolayer and from a second layer. The inability of the Langmuir,(1) Kisliuk,(2) or modified Kisliuk(3,4) models to fit the temperature dependence of the adsorption probability prompted the derivation of a model in which adsorption is forced through an extrinsic precursor. The difference in activation energies for desorption from the precursor state and migration of the extrinsic precursor, E-d' - E-m', is 5.4 kJ/mol, and the ratio of preexponential factors, k ' ((0))(d) / k ' ((0))(m), is 2 x 10(4). The difference in activation energies suggests that the extrinsic precursor makes about 10(5) site hops near the desorption temperature in search of a binding site. The high degree of corrugation in the gas-surface potential may be responsible for the relatively high barrier to migration in the extrinsic precursor.