=To further improve device technology, we need to observe and evaluate the structure and array of molecules on substrates. Here, we report the results of observing the porphyrin molecule isomers, tertiary-butyl-methoxy-phenyl-porphyrin (TBMPP) and tertiary-butyl-phenyl-vinylphenyl-porphyrin (TBPVPP), using a scanning tunneling microscope (STM). STM images of TBMPP showed differences in the distortion of the porphyrin ring. That apparently depended on the position of the methoxy group. In the case of the TBPVPP, the images showed cis- and trans-molecule structures that depended on the position of the vinyl group. We were able to observe molecular structures and identify isomers that are difficult to identify using chemical techniques. (C) 2004 Elsevier B.V. All rights reserved.