(57) Fin structure for a heat exchanger is fabricated by folding a strip of material back over itself at locations along the length of the strip such that integrally joining one of the folds on opposite sides are first confronting portions each of which in turn integrally joins by a further fold with a corresponding second confronting portion of the strip. Each pair of first and second confronting portions are processed to form a fin which has an interior space by joining their margins. This joining extends from the fold which integrally joins the first and second confronting portions substantially to the one fold. The one fold is deformed by displacing material from it more fully between the two fins, and preferably this is done at both ends of the fold. The displaced material strengthens the structure and spaces the fins apart.