Diffuse scattering of the thermodynamically stable decagonal quasicrystal (QC) Al70Ni15Co15has been measured at a synchrotron radiation source. Perpendicular to the unique ‘periodic’ direction there is an alternating sequence of ‘Bragg’ and (modulated) diffuse layers. Within the zeroth Bragg layer, strong diffuse streaks and diffuse satellite reflections, as well as very weak and slowly varying diffuse scattering, were measured up to high values ofQ. Satellites and diffuse streaks, already known from precession photographs, were explored in theq-scan mode yith a step width of 0.01 inhandk(pseudo-monoclinic setting witha*= 0.081 08 Å−1,b*= 0.081 09 Å−1in the QC plane,c* = 0.245 16 Å−1in the periodic direction,y= 72°). The positions, but not the intensities of all registered satellite reflections obey an overall tenfold symqetry around the Bragg reflection. The satellite vector magnitude ‒qs‒ = 3.3 × 10−2Å−1corresponds to a translational superperiod of the QC substructure with a 30 Å period. First- and second-order satellites were always observed only in pairs around the Bragg reflections, superimposed on the diffuse streaks through the Bragg reflection and the satellites. These observations may be interpreted by lamellar domains extended parallel to the unique axis. The full widths at half-maximum of the satellites are between 1.4 × 10−3and 2.0 × 10−3Å−1, corresponding to correlation lengths between 700 and 500 Å of the domain ordering. The diffuse streaks indicate faults in the regular sequence of the lamellar domains. An additional uniform diffuse background is observed, reflecting a high degree of disorder in the (projected) QC arrangement of the basic columnar-like structural elements.