A tool 20 includes an impermeable membrane 26 to be used as a vacuum bag. There is a leak detection film 28 between the bag 26 and the tool body 24 which is sensitive to oxygen so as to change physical appearance at leaks' locations. A woven or netting layer 40 should be placed between the film 28 and the tool body 24 to facilitate airflow therebetween (fig 4). Ideally the film 28 is transparent and oxygen permeable. Preferably the bag 26 is a flexible polymer. The film 28 may be bonded to or coextruded with the membrane 26, or applied on the tool body 24. Portions of the film 28 can be removable, eg in a grid pattern. The film can detect oxygen via additives such as nano particles. Ideally the film, or particles therein, luminesce under light of a given wavelength, and may illuminate a dye. The tool is for use in vacuum bag moulding and similar techniques. A seal 36 should be formed when using the device. Analysis of how the film changes over time may help identify a leak.