The trapping dynamics of methane and ethane onto preadsorbed methane-, ethane-, propane-, and n-butane-saturated layers on Pt(lll) at 30 and 50 K have been investigated using supersonic molecular beam techniques. At these low temperatures, multilayers of the incident molecule continue to build without the complication of desorption, and the weakly bound extrinsic precursor state is stabilized. The initial trapping of ethane into the extrinsic precursor state is enhanced to the same degree on all, the covered surfaces compared to trapping on the clean surface, but the initial trapping probability of methane, though enhanced by the adlayers, is more complex. The trapping probability of CH4 increases toward unity on all these surfaces as methane adsorbs and levels off at 0.96, independent of the type of adsorbate covering the bare surface. The angular dependence of the initial trapping probability indicates an increasing corrugation in the CH4-surface potential with increasing chain length of adsorbate; for comparison, methane traps on the clean surface with normal energy scaling,(1) whereas it traps on the n-butane-covered surface with near-total energy scaling.