The experimental conditions and precision of alternative methods of measuring isotopically exchangeable cobalt (Coi.e) were investigated. The values of Coi.e varied from 0.16 to 5.4 p.p.m. for 25 soils and were highly correlated with the sum of cobalt extracted by ammonium acetate and subsequently by quinol, and the relationships between these forms are discussed. The amounts of cobalt sorbed in the presence of calcium chloride by different soils varied markedly between and within soil groups. The bonding energy of specific sorption of cobalt did not vary significantly between soils after the first sites were filled. Cobalt sorption capacity for all soils studied was highly correlated with cobalt content and surface area and to a lesser extent with manganese and clay content and pH, but not with organic matter. When soils of high montmorillonite content were excluded, highly significant correlations were obtained only with manganese and cobalt contents and surface area. The initial slope of isotherms of isotopically exchangeable cobalt 60Co(soi1)/60Co (solution) and Coi.e may provide measures of the intensity and capacity factors, respectively, of cobalt availability to plants.