We present results of force measurements between hydrocarbon monolayer surfaces across n-alkanes (hexane, decane, and tetradecane). The interaction id qualitatively different from that of any previously system and, in particular, bears no resemblance to an oscillatory solvation force. Instead, the force is repulsive from about 2.5 nm, with the exeption of a shallow minimum just outside a force maximum at 0.8-0.9 nm. At smaller separations the force becomes attractive and there is a waek adhesion at contact. We suggest that the force law is due to a steric effect____a repulsive interaction originating in restrictions on chain conformations of the alkanes at small surface separations. This interaction is acessible via simple mean-field theories. The similarity of the liquid-liquid and liquid-surface interactions allows this to dominate over solvation effects. The results are of significance for interaggregate interactions in lamellar liquid crystals, microemulsions, and surfactant- stabilized dispersions.