Assuming jumping of interstitial impurities to neighboring sites, diffusivity was simulated using the Monte Carlo method based on the random walk theory. The jumping probability along a grain boundary was greater than that in the lattice of an inside grain. The crossing point of grain boundaries had a smaller jumping probability to neighboring sites on the grain boundary than that in the lattice. The results of simulation supported the grain boundary cross effect, proposed experimentally, on the grain size dependence of hydrogen diffusivity in an aluminum polycrystal. The diffusivity exhibited a peak at a critical grain size. The peak included two conflicting effects of grain boundaries: enhancement of diffusion along grain boundaries and suppression of diffusion by trapping at grain-boundary junctions. Monte Carlo Simulation of Grain Boundary Cross Effect on Hydrogen Diffusivity in Aluminum. M.Ichimura, Y.Sasajima: Japanese Journal of Applied Physics – 1, 1996, 35[1A], 186-90