Vibrational (EELS) and thermal desorption spectra (TDS) have been measured for the heterocyclic 5-membered ring molecules pyrrole (C 4H 4NH), furan (C 4H 4O) and thiophene (C 4H 4S) adsorbed on Cu(100) at 85–300 K. Adsorption is molecular and reversible with no evidence for dissociation. At low exposures, each molecule is weakly π-bonded to the CU(100) surface with the molecular ring parallel to the surface plane. The vibrational frequencies are essentially unperturbed relative to the condensed phases. At higher exposures, TDS and EELS results suggest two different models for adsorption prior to the onset of multilayer formation. Pyrrole and furan form a stable second layer (bi-layer) in which the molecular rings are inclined toward the surface normal. The bi-layer and the π-bonded monolayer can be separately isolated by thermal treatment. Thiophene, however, appears to increase the monolayer coverage by “compression” on the Cu(100) surface, with the average tilt angle of the rings changing with coverage. At higher exposures at 85 K, multilayer condensation is initiated for all molecules. Although the molecular frequency shifts for these adsorbed molecules are very small relative to the condensed phases, evidence for the different adsorption models is obtained from TDS studies and by application of the surface dipole selection rule. The case for the structure of the bi-layer of furan on Cu(100) is strongly supported by the known low temperature crystal structure in which alternate layers of rings are inclined to one another. The weak nature of these adsorption systems and lack of dissociation is consistent with previous work on weak molecular adsorption on copper surfaces.