A study of the influence of adsorbate solution pH on the packing density and electrochemical reactivity of (4-pyridyl)-hydroquinone (4PHQ) molecules adsorbed at a Pt(III)thin-layer electrode (TLE) is reported. This compound affords the opportunity to study a reversibly electroactive adsorbate molecule which can undergo protonation at one of its most favorable points of attachment to the electrode surface. The packing density of 4PHQ increases abruptly when the pH is increased above 2 and exhibits a plateau above pH 5. A fraction of the adsorbed 4PHQ is reversibly electroactive, consistent with the expected two-electron, two-proton quinone/hydroquinone half-reaction. The fraction of electroactivity increases sharply above pH 2, and approaches 100% above pH 5. UV-visible spectra of 4PHQ versus pH indicate that the protonation-deprotonation equilibrium of the pyridine moiety is centered at about pH 6, while the adsorbate orientation transition is centered near pH 3, probably due to the strong driving force of bonding between Pt and the unprotonated pyridyl nitrogen atom. The packing density of 4PHQ is relatively insensitive to adsorbate concentration in the range examined here (10(-5) to 10(-3) M). These results indicate that 4PHQ adsorbs primarily in an N-bonded vertical orientation (N-eta(1)) at pH greater than 5, while a ring-bonded "horizontal" orientation (eta(14)) predominates at pH 0-2, with a transitional region at pH 2 to 5. The vertically oriented state is reversibly electroactive, while the horizontally oriented state is electroactive only toward irreversible oxidation at extremely positive potentials. Voltammetric characterization of the Pt(III) TLE surface indicates that the surface consists primarily of well-ordered Pt(III) along with a small fraction of surface area which has been abraded during contact with the Pyrex glass enclosure surrounding the thin-layer of solution.