We have succeeded in monitoring the reaction of an acetate-covered TiO2(110) surface by scanning tunneling microscopy (STM) operated in a temperature-jump mode. Acetate ions were individually resolved at high temperatures, where they started decomposing, due to restricted mobility on the metal-oxide surface. The microscopic kinetics of the decomposition reaction determined by molecular counting in sequential STM images agreed with the macroscopic rate law deduced from a thermal desorption study. The present study demonstrated the ability of STM for monitoring reaction of ionic intermediates liberated on metal-oxide surfaces.