Quantitation by use of Auger spectroscopy and cyclic voltammetry of molecular layers adsorbed at Pt(ll1) and Pt(100) surfaces from aqueous electrolytes is examined in this work for the following compounds: hydroquinone (HQ); phenol (PL); catechol (CT); 3,4_dihydroxyphenylacetic acid (DOPAC); L-3,4_dihydroxyphenylalanine (DOPA); L-tyrosine (TYR); L-phenylalanine (PHE); nicotinic acid (NA); 2,5dihydroxy-4-methyl-benzyl mercaptan (DMBM); thiophenol (TP); benzylmercaptan (BM); 3-thiophene carboxylic acid; and 2,5,2',5'-tetrahydroxybiphenyl (THBP). Two independent methods of measurement of packing density based upon Auger spectroscopy, and one based upon cyclic voltammetry are employed and the results compared. Voltammetric oxidation/reduction of adsorbed layers formed from these compounds at Pt surfaces in aqueous electrolyte is found to be essentially the same whether carried out before or after lengthy evacuation. Therefore, the results of surface spectroscopy in UHV are directly applicable to the liquid-solid chemistry and electrochemistry of these adsorbed compounds. Packing densities measured by means of two Auger SnectroscoDic methods were in good agreement with each other and with the -_ voltammetric measurements. SH BM THBP Scheme 1.