The lifetimes of heterogeneous catalysts in many widely used industrial processes are determined by the loss of active surface area. In this context, the underlying physical sintering mechanism and quantitative information about the rate of sintering at industrial conditions are relevant. In this paper, particle migration and coalescence in nickel steam reforming catalysts is studied. Density functional theory calculations indicate that Ni-OH dominate nickel transport at nickel surfaces in the presence of steam and hydrogen as Ni-OH has the lowest combined energies of formation and diffusion compared to other potential nickel transport species. The relation between experimental catalyst sintering data and the effective mass diffusion constant for Ni-OH is established by numerical modelling of the particle migration and coalescence process. Using this relation, the effective mass diffusion constant as a function of temperature is determined from experimental data to D _Ni-OHK_Ni-OH = 7.7 × 10^-6 m² s^-1 bar^-0.5exp((- 137 kJ/mol)/RT). With this work we are able to bridge the gap between sintering in macroscopic catalysts and microscopic particle diffusion.