The thermodynamic parameters and rheological properties of a synthetic glycolipid with a gentiobiose (O(beta-D-glucopyranosyl)-(1-->6-beta-D-glucopyranoside)) headgroup have been studied at the air/water interface. The phase transition entropy and latent heat were estimated by film balance experiments at various temperatures, whereas dynamic surface moduli (loss and storage modulus) were measured by an interfacial stress rheometer under well-defined thermodynamic conditions. The impact of glucosidic bonding was systematically studied by comparing the measured results to those from the lipid with a lactose (O-(beta-D-galactopyranosyl)-(1-->4-beta-D-glucopyranoside)) headgroup, which possesses the similar monosaccharide composition. We found that the "bent" 1-->6 junction of gentiobiose reduces the lateral intermolecular cooperativity and fluidizes the films in comparison to the lactose lipid monolayer.