Diffuse layers corresponding to an 8 Å superperiod along the 'periodic' axis of decagonal Al-Ni-Co phases were investigated by in situ X-ray and neutron diffraction at temperatures up to 980 o C. The diffuse layers consist of an unstructured diffuse background, diffuse bands, and diffuse maxima which (almost) all vanish above 900 o C, but show a different temperature dependence in detail. A tentative scenario of the related structural ordering process during cooling from high temperature is developed. A 1D-columnar cluster model is compared to the experimental results.