X-ray reflectivity investigations were performed on organic light emitting diodes (LEDs) with the purpose of characterizing those properties which control the performance of the devices. These are the thickness of the electrodes and of the active organic films. the roughness of the layers and of the substrate, as well as the internal structure of the organic films. Based on the experimental results and corresponding simulations on multilayer polymer/metal structures, it has become obvious that X-ray reflectometry constitutes anon-destructive technique for characterizing light emitting diodes with a complex multilayer architecture.