The composition of the active layer of a benchmark functional glass/ITO/[P3HT:PCBM][1.1:1]/Al organic solar cell has been studied by neutron reflectometry (NR) and hard X-ray photoelectron spectroscopy (HAXPES). Thermal annealing was performed in several steps and NR and HAXPES were recorded for every temperature. By fitting the NR results to a model composed of several layers, the scattering length density (SLD) distribution through the sample was obtained, and from this SLD profile, the evolution of the composition of the active layer as a function of temperature was established. For the outer layers, HAXPES results confirm the composition evolution. The results show that PCBM tends to segregate reducing the initial concentration of PCBM in the central part of the active layer and increasing its concentration towards both interfaces. The effect of the Al electrode as studied by HAXPES on the nearest zone of the active layer (up to 50 nm) is to stabilize the P3HT depletion in this area, an effect which is not affected by thermal annealing.