X-ray standing wave measurements, along with local-density molecular c
luster calculations, were used to determine the surface structure of t
he low-coverage Bi/Si(001) surface, The 2x2 Bi phase, predicted by the
ory to be the stable low-coverage structure, was found experimentally
to be a metastable phase and was observed to have a thermally activate
d irreversible phase transition to the stable 1x2 phase. These measure
ments represent another approach for investigating the kinetics of sur
face reactions.