The adsorption states of thiophene on the Si(1 00)-(2 x 1) surfaces have been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT). Comparison of bias dependent images of STM with DFT results indicate that room temperature adsorption at low coverages leads to four types of configurations; 2,3-dihydrothiophene-like and 2,5-dihydrothiophene-like species as di-sigma bonded structures, twisted bridge-like and tight-bridge tetrahydrothiophene-like species as tetra-sigma bonded structures. Conversion from the di-sigma structures to the tetra-sigma ones occurs at room temperature. The population ratios of the configurations observed with and without STM scans confirm that this conversion takes place thermally. (c) 2007 Elsevier B.V. All rights reserved.