The interfacial behaviour of two N,N'-tetrasubstituted malonamides, N,N'-dimethyl-N,N'-diphenyl malonamide (DMDPHMA) and N,N'-dimethyl-N,N'-dicyclohexylmalonamide (DMDCHMA), in the liquid-liquid (L/L) extraction of iron(III) from hydrochloric acid solutions, was studied. The experimental results obtained validate the absence of third phases for these systems. The equilibrium adsorption constants and surface excess concentrations, estimated by the Szyszkowski model, explain the experimental extraction efficiency patterns for both malonamides in the two diluents tested, 1,2-dichloroethane (1,2-DCE) and toluene. A solvation type mechanism is in accordance with all the results obtained, accounting for the structural and diluent effects observed. (C) 2013 Elsevier Inc. All rights reserved.