The greater part of younger halogenetic polder soils in the 6 th and the 7 th section of Kojima Polder consist of heavy clay and have sticky, puddled structure and are quite impervious to water. It was found that the younger polder soils contain a large amount of the soluble salts, and of the exchangeable bases, the MgO and Na2O are more prominent. As is well known, the abnormal composition of exchangeable bases is considered to be the major factor of the poor physical condition of these polder soils. The experimental work reported in this paper deals with the problem of the effects of iron sulfide which has been formed in polder soils under an. anaerobic condition on the solubility of added calcium carbonate and the composition of exchangeable bases after drainage. Kojima heavy clay soil and Fukuda loamy soil were chosen for the test and calcium carbonate and starch were added to each soil sample. All of the soil samples were preliminarily incubated in water-logged condition for about one year and then incubated at 30℃ under the condition of the moisture content of field capacity for several weeks. Changes of pH and soluble salts by reduction and oxidation of samples were determined. (Table 1) The comparison of exchangeable bases between original. soils and incubated samples was made. (Table 2 & 3) The incubated samples were leached by carbonate water for five times. Successive and total losses of CaO, MgO and SO3 in every leachate were determined. (Table 6 & 7) The comparison of exchangeable bases between incubated samples and leached samples was made. (Table 2 & 3) The results obtained may be summarized as follows: (1) The reaction of water-logged standard samples is alkaline, pH ranging from 7.2 to 10.3. After incubation, iron sulfide oxidizes and most of these samples show strong acidities, ranging from pH 4.3 to 6.1, and water-soluble sulfates and calcium increased greatly. After leaching, the exchangeable bases are replaced by hydrogen and percentage base saturation is decreased to a maked extent in every soil. (2) The reaction of water-logged samples, to which calcium carbonate was added is more alkaline as compared with standard samples. The pH values of these samples decrease to some degree after incubation but they remain neutral to slightly alkaline, ranging from pH 7.0 to 8.0. In the incubated samples, there is a marked increase of water-soluble sulfates and calcium and exchange calcium, but exchange Mg, Na and K are decreased. It appears that during incubation most of the iron sulfide oxidizes; the H2SO4 formed reacts with the CaCO3 and CaSO4 is formed; the CaSO4 rapidly changes the sodium clay into calcium clay. During the leaching process, the solution of CaCO3 and the exchange of Ca for Na and Mg proceeds to a large extent; consequently a considerable part of exchange Na and Mg are replaced by Ca in the final situation. (3) The greater part of soluble calcium and magnesium are leached in the first leachate from the standard samples but considerable quantities are leached successively in every leachate from the samples to which CaCO3 is added. A far greater SO3 outgo occurs from the first leachate of standard samples than from the CaCO3 added samples. It appears that CaCO3 retards the oxidation of iron sulfide during incubation.