The molecular adsorption of thiophene and 2,5-dimethylthiophene on Ag(111) surfaces is studied using a cryogenic scanning tunneling microscope (STM) operating at 120 K. Thiophene molecules are observed to adsorb preferentially at step edges, occupying the upper step edge at lowest coverage, and additionally occupying the lower step edge at higher coverage. The adsorption site and rotational orientation of 2.5-dimethylthiophene were identified. Molecules at step edges art : observed to cluster, indicating an attractive interaction between molecules at step edges. As the coverage increases, a phase transition corresponding to a transition from a planar to a tilted geometry is observed.