The structure of a p-xylene adlayer formed on Rh(111) in ultrahigh vacuum (UHV) at room temperaturewas investigated by using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED).STM images revealed that p-xylene formed a highly ordered adlayer on Rh(111) in UHV. From the STMobservation and the LEED measurement, the adlayer structure was determined to be c(2√3 × 4)rect,which was reported previously also for the adlayer on Rh(111) in HF solution. Time-dependent STMallowed direct observation of the dynamics of the self-ordering process. Small ordered domains of adsorbedp-xylene molecules appeared and gradually grew in size on terraces. The domain size was found to increasein a few minutes under the exposure to p-xylene. The average domain size was ca. 20 nm.