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We present two recent applications of lattice-gas modeling techniques to electrochemical adsorption on catalytically active metal substrates: urea on Pt (100) and (bi)sulfate on Rh(111). Both systems involve the speci c adsorption of small molecules or ions on well-characterized single-crystal electrodes, and they provide a particularly good t between the adsorbate geometry and the substrate structure. The close geometric t facilitates the formation of ordered submonolayer adsorbate phases in a range of electrode potential positive of the range in which an adsorbed monolayer of hydrogen is stable. In both systems the ordered-phase region is separated from the adsorbed-hydrogen region by a phase transition, signi ed in cyclic voltammograms by a sharp current peak. Based on data from in situ radiochemical surface concentration measurements, cyclic voltammetry, and scanning tunneling microscopy, and ex situ Auger electron spectroscopy and low-energy electron di raction, we have developed speci c lattice-gas models for the two systems. These models were studied by group-theoretical ground-state calculations and numerical Monte Carlo simulations, and e ective lattice-gas interaction parameters were determined so as to provide agreement with the experimental results.