A method of making an aerofoil comprises the steps of providing first and second skin panels and first and second web-forming membranes. The web-forming membranes each have a series of elongate slots formed longitudinally therein so as to define a series of alternating wide and narrow strips. The wide strips of one membrane are arranged against the narrow strips of the other membrane and the skin panels are arranged either side of the two web-forming membranes. Parts of the web-forming membranes and parts of the skin panels are treated with a release layer and the sub-assembly is compressed and heated so as to effect diffusion bonding between those parts of the membranes and skin panels which are not treated with the release layer. The sub-assembly is then heated and inflated so as to draw the first and second skin panels apart and that, in turn, causes the web-forming membranes to form webs internally.