We report our result on the characterisation of buried polymer-polymer interfaces in thin polymer films using positron beam based Doppler and life-time techniques. Polystyrene(PS) bilayers(PS/PS) were prepared by flotation method from spin coated films(thickness similar to 500 nm), The interface structure was modified by annealing the bilayers above the glass-transition ternperature(Tg) for predetermined times. Measurements in 'as prepared' and annealed samples were carried out as a function of depth across the buried interface. The result are discussed.