The adsorbed slate and thermal decomposition of formic acid (HCOOH) on the Si(100)(2 x 1) surface covered with a monolayer of K have been studied by using electron energy loss spectroscopy and thermal desorption spectroscopy. Three types of formate species are observed : (1) three-dimensional HCOOK islands, (2) HCOOK of the first layer, and (3) HCOOSi adsorbed in the K-free patches. HCOOK of the first layer is thermally stabilized, while thermal decomposition of HCOOSi is promoted by the presence of K. Mechanisms of the formation of HCOOK islands and HCOOSi species are discussed. Comparisons of HCOO on Si(100)(2 x 1)-K and clean Si(100)(2 x 1) surfaces are made.