SUMMARYSorption isotherms were obtained by a continuous‐flow method for the interaction of hydrazine hydrate with freeze‐dried hydrogen‐, aluminium‐ and calcium‐exchanged humic acid preparations in aqueous suspensions at pH 4. The application of the continuous‐flow method is outlined and the fit of the isotherms to three sorption equations is examined. Microcalorimetry was used to measure directly the differential enthalpies of reaction as functions of sorptive concentrations. Exchange by hydrazinium ions of the exchangeable cations on the humates, chemisorption by interaction of hydrazine with humate carbonyl groups, and non‐specific sorption involving weakly‐ and strongly‐held hydrazinium ions and hydrazine molecules are shown to be the major sorption processes in these systems. The relative importance of each of these processes for the different hydrazine/humate systems is discussed, and the influence of the exchangeable cation on humate reactiv