Supersonic molecular beam experiments were used to probe the temperature dependence of adsorption of methane and ethane into adsorbed layers of methane, ethane, propane, and n-butane on Pt(111). Because adsorption must proceed via a transient state from which either desorption or conversion to a bound state may occur, the adsorption probability is, in general, temperature dependent. As the surface temperature is lowered below 75 K increasing amounts of methane adsorbs into an alkane-covered surface to form a mixed adlayer. Methane forms multilayers on methane, ethane, propane, and butane adsorbates below 35 K; ethane forms multilayers on the same adsorbates below 63 K. The condensation temperature of methane and ethane into multilayers is independent of the type of preadsorbed alkane and occurs well below the normal boiling or melting point of each species. At a given temperature between 75 K and 35 K the steady-state uptake of methane (or ethane between 63 K to 120 K) into the first layer decreases with increasing carbon chain length of the preadsorbed alkane, reflecting the lesser availability of adsorption sites due to more extensive site blocking by the larger alkanes. The temperature-dependent sticking probability and steady-state uptake indicate an extrinsic precursor mechanism to molecular adsorption on all the alkane-covered surfaces. For methane adsorption on ethane-saturated Pt(111), the difference in activation energies for desorption and migration to a binding site from the extrinsic precursor, E-d' - E-m', is 5.03 kJ/mol and the ratio of preexponential factors, k(d)' ((0))/km ' ((0)), is 4.1 x 10(3).