Nickel-base superalloys with gamma/gamma' microstructure are state of the art material for single crystal turbine blades. They can be coated with aluminum and subsequently diffusion annealed in order to increase their oxidation resistance. EBSD and EDX measurements were used to determine the different phases in the diffusion zone and to gather information about the diffusion and phase transformation mechanisms. It was found that during heat treatment, aluminum diffuses from the coating into the L1(2) ordered Ni3Al substrate crystal and forms a complex precipitation zone which consists of several different phases. A large amount of crystals in this zone belongs to the B2 ordered NiAl phase. These crystals show a clear and sharp orientation relationship with the Ni3Al substrate. It is concluded that nucleation of the new crystals in the substrate proceeds by a form of coherent (or semi-coherent) precipitation. This assumption is supported by calculations of the lattice misfit between the parallel crystal planes in both phases.