Improvements in machines for mounting valve-type sacks (4a, b, c) on filler ports (51), said machines comprising devices for withdrawing one sack (4a, b, c) at a time from a pile of horizontally lying sacks, for making at least part of the withdrawn sack vertical, and for then feeding it to said port, in which said devices comprise means for withdrawing the sack (4) and making at least part of it vertical, these means being provided with a sucker arrangement (20) forming part of a penumatically controlled articulated parallelogram (15, 17, 12, 13) which acts on that end (4a) of the sack (4) at which the valve (4c) is provided and transfers the withdrawn sack to a sack support and guide means (22, 25) comprising a stationary component (22) and a component (25) movable away from and towards said stationary component, said components (22, 25) comprising rotatable means (24, 28) arranged to cooperate with motor means (40) to feed the sack towards the filler port (51).