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Aluminium brazing is a method used for joining Al-parts by applying liquid alloys which have lower melting points than aluminium. The main task during Al brazing is the successful elimination of the oxide layer which forms on the free Al surfaces even at low oxygen partial pressures. This layer is continuous resulting in a poor wetting ability of the surfaces to be joined. Hence, the elimination of this oxide layer is inevitable for successful brazing. This can be realised by the application of suitable flux material dissolving the oxide layer, simultaneously ensuring temporal protection against the reconstruction of oxide layer during the heating period. In this paper, the CO2 laser beam is applied as an energy source which yields rapid local surface melting. However, the absorption ability of Al is very low around the characteristic wavelength of this energy beam. In order to improve the absorption ability, the composition of NOCOLOK® flux was properly modified by Si powder addition. The absorption ability of the flux-covered surfaces was determined indirectly by measuring the emission coefficient at the wavelength of CO2 laser (10.6 μm). On the basis of surface emission measurements, the flux-composition can be successfully tailored according to the technical requirement dictated by the joining of tubes or sheets with different diameter and wall-thickness. [Copyright &y& Elsevier]