The optical and structural properties of an electrolytic black cobalt absorber have been investigated. Deposited on copper-coated steel substrates, this coating is used as a selective surface in the vacuum environment of the Philips evacuated receiver tubes. Due to the infrared absorption of the porous black cobalt material, the thermo-optical properties are considerably dependent on plating time. In addition, the morphology of the coatings is significantly influenced by the surface roughness of the substrates. The results of the optical and analytical studies are discussed in terms of a structure model, which mainly assumes the deposit as a granular composition of particles consisting of a cobalt-rich phase. Due to the high thermal stability of this phase in an inert atmosphere, high vacuum-annealing of the coatings at temperatures of 450°C results in essentially unchanged optical properties, although some filling material will be decomposed and converted to cobalt oxide upon heating. Thus, the material is shown to be well suited for the application even in high temperature evacuated tubular collectors, provided the annealing/degassing process is carried out under inert conditions. Otherwise, the attack of oxygen may cause severe degradation of both the coating and the substrate.