In situ treatment technologies, including chemical treatment by Fenton's reaction, are expected to be effective for site remediation of polluted soil. To get basic data on the decomposition rate of trichloroethylene (TCE) by Fenton's reaction using iron powder, we investigated the influence of pH and dissolved Fe ion concentration on the decomposition rate of TCE in the presence and absence of buffer solution to maintain pH value. The decomposition rate depended on hydrogen peroxide concentration, and the maximum rate was observed around concentration of 88 mol/m(3). This finding was explained by the consumption of hydroxyl radicals by a competing reaction with hydrogen peroxide against TCE decomposition by the hydroxyl radicals produced from hydrogen peroxide. Under this condition, the most rapid dissolution rate of iron, i.e., the maximum increasing rate of Fe ion concentration, and the most rapid reduction in pH value were observed. The results with buffer solution showed that dissolution rate of iron was highly dependent on pH, and it was suggested that TCE decomposition was accelerated by the increase of Fe ions, rather than directly by the lowering of pH. The initial lag period in the TCE decomposition with iron powder, in contrast to the rapid initial reaction in the case of Fe solution, was also explained by the relatively slow dissolution of iron and relatively fast rate of Fenton's reaction with ferrous iron to produce hydroxyl radicals. The concentration of Fe ions was estimated and suggested to be much higher and the pH lower in the film around iron powder than in the body of the liquid. Also, the transfer of TCE from the liquid body to the reaction area in the film was suggested to be accelerated by its decomposition.