This paper describes an original method to measure the plutonium self-diffusion coefficient in the mixed oxide U0.55Pu0.45O2±x. This method is based on using 242Pu as a tracer atom. A thin film of the tracer was deposited on the well-polished surface of the samples and then diffusion annealings were performed from 1500°C to 1700°C, in an Ar-H2 5% atmosphere. The oxygen potential was fixed at-395 kJ.mol-1. After annealing, the 242Pu self-diffusion profiles were established by means of secondary ion mass spectrometry (SIMS). The 242Pu concentration profiles were determined by assessing the relative U, Am and Pu ionisation yields according to the experimental parameters. The choice of favourable experimental conditions and the relevance of the resulting concentration profiles are discussed at length