We probe the mechanical shear and compression properties of hydrogen-bonded polymer multilayers directly assembled at the oil−water interface using interfacial rheology techniques. We show that the polymer multilayers behave mechanically like a transient network, with elastic moduli that can be varied over 2 orders of magnitude by controlling the type and strength of physical interactions involved in the multilayers, which are controlled by the pH and the hydrophobicity of the polymer. Indeed, the interplay of hydrogen and hydrophobic interactions enables one to obtain a tighter and stronger network at the interface. Moreover, we show how a simple LBL process applied directly on emulsion droplets leads to encapsulation of a model oil, dodecane, as well as perfume molecules. P olymer capsules are used in a wide range of applications such as cosmetics, pesticides, food and detergency, where protection, transport and delivery of active species are needed. 1 Capsules prepared by the layer-by-layer assembly (LBL) 2 of polymer multilayers onto a colloidal template is a powerful method to produce polymer membranes with controlled thickness, porosity, and permeability. 3−6 Such capsules are a good alternative to other technologies based on cross-linking reactions at the interface of oil droplets 7 that may raise concerns about potential residuals in suspensions of capsules. However, in this LBL process, the core has to be dissolved to obtain a hollow capsule that then needs to be filled with active species. Recently, to circumvent these issues, stable O/W emulsions were prepared using a polymer surfactant and then the LBL method was used to build in situ a polymer membrane on the oil droplets. 8−10 In addition to stability and encapsulation properties, most applications require precise control of the mechanical properties of the capsules to resist environmental stresses and achieve suitable encapsulation/ release properties with respect to the desired application. For example, in detergency applications, fragrance capsules need to remain stable during storage in end-use products like liquid detergents, but the perfume molecules should be delivered to clothes at some key stages of the whole washing/drying process. This example can be extended to other fields of capsule application and shows the importance of an appropriate control of the mechanical properties of the capsules membrane. To the best of our knowledge there are no rules to guide the capsule design, mainly because of a lack of experimental methods available to easily measure the mechanical properties of capsules. 11−14 In this letter, we probe the shear and compression mechanical properties of hydrogen-bonded polymer multilayers directly assembled on a single millimetric oil droplet in water and at a flat oil−water interface. We show that the elastic moduli of the multilayers can be varied over orders of magnitude by using an interplay of hydrogen and hydrophobic interactions between the layers. Moreover, we show how a simple LBL process allows the assembly o...