The adsorption of phenyl hydroquinone (PHQ) on graphene surfaces at the liquid-solid interface is investigated revealing a flat orientation and two different vertically adsorbed states of PHQ on graphene nanoplatelets (GNPs), namely edgewise or endwise adsorption. The transition between these states is driven by increasing concentrations of PHQ in solution leading to increased absolute coverages on the graphene surface. At low adsorbate concentrations (≤21 mM), the adsorption process is also shown to be Langmuirian with an adsorption constant of (9.5 ± 0.2) mM-1. Independent measurements are conducted using a single particle electrochemical technique to confirm the surface coverage of PHQ on GNPs at low concentrations, showing excellent agreement with the UV-Vis studies.