Using conjugated polymers as the active materials in electronic and optoelectronic devices opens up the possibility of fabricating all-polymer devices, such as active matrix displays, photovoltaic cells and integrated circuits, using low-cost solution processing technologies. The operation of all of these devices depends crucially on processes occuring at polymer-polymer interfaces. To date we have performed a series of specular reflectivity experiments on both dielectric/conjugated polymer interfaces and conjugated polymer/conjugated polymer interfaces (heterojunctions) in parallel with device fabrication, in an attempt to determine the interaction between the physical structure of interfaces and device performance. Our aim here is to extend this characterisation by using off-specular reflectivity to probe structure in the plane of the interface.