Self-diffusion in thin nanocrystalline Pt films was investigated using secondary ion mass spectrometry. Our experiments are motivated by recent investigations on stress relaxation where self-diffusion of Pt is supposed to play an important role, especially at temperatures below 250 °C and annealing times of a few hours. For the diffusion experiments, double layers of ^natPt/¹⁹⁴Pt were deposited on oxidized silicon wafers using ion beam sputtering. At 180 °C no significant diffusion induced broadening of the profiles could be observed even after an annealing time of 64 h. However, the concentration of ¹⁹⁵Pt in the top layer decreases slightly after an annealing time of 16 h and remains constant for higher annealing times. At 600 °C a broadening of the profiles was observed after an annealing time of 5 minutes. From our results we conclude that at 180 °C only atoms in the grain boundaries are mobile. After about 16 h the isotopes in the grain boundaries are completely interdiffused. From the change of the ¹⁹⁵Pt concentration in the top layer we estimate the amount of grain boundary phase in the Pt films to be about 5 %. The broadening of the profile after annealing at 600 °C is attributed to bulk diffusion.