SummaryWe studied the properties of the soluble and dispersed compounds of Cu, Mn, Co, Ni, Pb, Zn, and Cd formed by the action of aerobically decomposing plant matter on the respective metal oxides. The metals were mobilized partly in association with colloidal humified organic matter, and partly in true solution as complexes that seemed to be anionic.In the presence of a clay soil there was no net mobilization of colloidally bound Cu, but the dialysable Cu complex was not appreciably sorbed by the mineral colloids and was leached from the reaction mixture.The metals were not precipitated under alkaline conditions from the dialysable complex forms. Material with similar complexing properties was found in the dialysable fractions of a soil organic matter extract, of water squeezed from a raw peat, and of laboratory lysimeter solutions from a podzol underCalluna.Below about pH 6 the exchange of Cu on a soil clay was not affected by the presence of colloidal decomposition products of lucerne. With Co, Ni, and Zn the corresponding pH value was about 4, and the critical value for Cu in the presence of colloidal soil organic matter was also about 4. Below these values the metal and organic matter sorption curves were diametrically opposed so that under these conditions Cu is apparently not strongly bonded to colloidal organic matter.