To clarify the chemical form of soluble cadmium and copper in arable soils, the molecular weight distribution and ion composition of water-soluble cadmium and copper in soils were investigated. The solution extracted with water from soil was fractionated by ultrafiltration and then cadmium and copper in 4 molecular weight size classes were determined. About 50% of water-soluble cadmium was present in the fraction with a molecular weight of 500 or less, 12-25% in the fraction with a molecular weight of 500-300,000 and 25-37% in the fraction with a molecular weight of 300,000 or more. For copper, 4-19% was present in the fraction with a molecular weight of 500 or less and 40-75% was present in the fraction with a molecular weight of 10,000-300,000 and 300,000 or more. The water-soluble heavy metals were also fractionated with ion exchange resin. More than 90% of cadmium was detected in the cation fraction. Conversely, approximately 80% of copper was detected in the anion fraction. These results indicate that water-soluble cadmium in soil is present as divalent cations, and copper is dissolved in a form bonded with substances such as organic matter in soil. To understand the difference in the mobility of cadmium and copper in soils, a plastic container was filled with moist soil and the soil moisture was allowed to evaporate only from the upper part of the container. Water-soluble cadmium was concentrated to a level of 68.5 times the background concentration at the top of the soil and copper was concentrated to 6 times the background concentration. Such a difference in the mobility was discussed in relation to the chemical form of soluble cadmium and copper.