The measured heat of adsorption for the oleophilic rust inhibitor sodium dinonylnaphthalenesulfonate, at the iron oxidelcyclohexane interface was found, to be 42.3 Kj/mole at. 30°C. In systems containing trace amounts of water, the measured heat of adsorption was found to decrease with increasing water concentration. An investigation of the effect of water on sulfonate adsorption indicated that (1) a complex equilibrium involving adsorbed and micellar water exists and (2) water penetration of a sulfonate monolayer is a facile process. These findings contradict this laboratory's earlier suggestion of a barrier type mechanism and indicate the importance of bulk fluid adsorption to form a mixed monolayer in the mechanism of rust protection.