The scanning tunneling microscope has been used to distinguish a series of related molecules (naphthalene, azulene, and various methylazulenes) on Pt(111) on the basis of their shape. Typical lower resolution images are used to measure the relative sticking coefficients and relative diffusion rates of the molecules. The orientations and binding sites of several of the molecules on the surface are also assigned. Tip dependent higher resolution images can show internal structural details on molecular species which have low diffusion rates.