The electrochemical hydrogenation and oxidation of p-dihydroxybenzene (hydroquinone) chemisorbed at polycrystalline Pt, well-ordered Pt(111) and disordered Pt(111) electrodes were studied by differential electrochemical mass spectrometry (DEMS). For comparative purposes, benzene was investigated at polycrystalline Pt. Anodic oxidation yielded only CO2 as the volatile (DEMS-detectable) product. However, at least three oxidation cycles were necessary for exhaustive oxidation; this indicates that: (i) non-volatile products were generated in the first cycle, (ii) these products either stayed adsorbed or, in part, were re-adsorbed during a cathodic scan into the double-layer potential region, and (iii) these species were further oxidized on subsequent anodic scans. Electrocatalytic hydrogenation of hydroquinone at polycrystalline Pt followed two parallel (not sequential) paths to generate benzene and cyclohexane: the "branching ratio" was heavily in favor of the latter product. The adsorbate can be displaced by CO at potentials in the double layer region. Since no volatile species was observed during this process, the adsorbate is not already reduced; e.g., to benzene. On well-ordered Pt(111), no cyclohexane was produced and only a minuscule fraction of benzene was observed; however, quantitative desorption of (unidentified) non-volatile organic material took place at the negative potential. The disordered Pt(111) surface behaved more like the polycrystalline rather than the monocrystalline surface. The H(2)Q-hydrogenation reaction proceeds more readily on Pt with surface steps and kinks.