Abstract: The subject of the present invention is an improved leaf packet useful in forming a spiral filtration module, the improved module which results from utilization of the leaf packet and a method of preparing the leaf packet. The membrane sheet utilized in forming a spiral filtration module is fused and densified in the area of its fold to increase the sheet density in this area and thus provide protection against mechanical failure. The densification includes the application of heat and pressure to the membrane surface of the sheet so as to densify the membrane surface and seal it against the penetration of feed stream liquid while also densifying the membrane, the backing and an optional reinforcing strip (if utilized).