The microstructure and microchemistry of two Pt modified and one plain aluminide diffusion coating (RT22, SS82A and PWA73) in the as-coated condition and after heat treatment at 1 050 degreesC for times up to 5 000 h have been investigated by means of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDS). It was found that high concentrations of Pt in the outer zone of the coating causes precipitation of alpha -W by lowering the solubility of W in the beta -NiAl matrix. Low concentrations of Pt (up to about 4 at%) may have a direct or indirect stabilising effect on beta -NiAl on the Al-deficient side. The interdiffusion zone between the coating and the substrate was observed to grow during heat treatment, due to inward diffusion of Al. A model where the growth of the interdiffusion zone is connected to the inward flux of Al is proposed.