Abstract: A leak-proof filling valve for a paper sack is formed by folding a composite sheet formed from Kraft paper 1 on top of which is laid relatively flexible polythene 2. The folded sheet is secured between end flaps 10A, 12 of a paper sack. The sack is charged with material 16 through a mouth 15 and passes through a short inner channel between parts 1C, 1D, and a longer outer channel between parts 2A, 2B. Reverse flow between parts 2A, 2B is prevented by material which percolates into pocket 17, assisted by the different flexibilities of the parts 1A, 2A. Any material which nevertheless does pass into space 18 is prevented from escaping between parts 1C, 1D by accumulation of material in pocket 19 this being assisted by the different flexibilities of parts 1C, 2C.