Scanning tunneling microscope (STM) is used to study the reaction kinetics of Sb4 on Si(001). Four distinct types of precursor states are observed which can be converted to the final state of chemisorption by either thermal annealing or by a STM tip-induced conversion process. The STM tip can also reversibly rotate the orientation of individual Sb dimers, as well as displace them. A mechanism based on the polarizability is proposed for die interaction between the STM tip and the Sb clusters. A new image-anneal-image method is developed for studying surface kinetics in which the same adsorbates are imaged before and after thermal annealing. This allows direct observations of the thermal reaction of individual adsorbates while avoiding potential STM tip effects on the reaction. Using this method, the detailed reaction path of Sb4 on Si(001) is directly determined. The precursors are found to be immobile before conversion, contrary to the popular conception about precursors.