A unit for applying at least one pre-formed opening device onto a hole of a web of a packaging material comprising: - advancing means for advancing said web along a path; and - at least one application station, which is fed, in use, with said pre-formed opening device and seals, in use, said pre-formed opening device on said hole of said web of packaging material, said application station comprising o a sonotrode which can be activated to cause the sealing of said pre-formed opening device to said web of packaging material; and o an anvil, which is adapted to co-operate with said pre-formed opening device, on the opposite side of said sonotrode with respect to said web, during the sealing of said pre-formed opening device to said web, characterized in that - said sonotrode is movable along a first direction between ∘ a first rest position, in which it is spaced apart from said web; and ∘ a first operative position, in which it cooperates with a first side of said web at said hole, and in which it is activated to cause the sealing of said pre-formed opening device; - said anvil is movable along a second direction transversal to said first direction between ∘ a second rest position, in which it is spaced apart from said web; and ∘ a second operative position, in which it exerts a counter-pressure against a second side, opposite to said first side, of said web at said hole; and - said sonotrode and said anvil are arranged, in use, at the same time, in respective said second operative positions, so as to seal said pre-formed opening device to said web. - advancing means for advancing said web along a path; and - at least one application station, which is fed, in use, with said pre-formed opening device and seals, in use, said pre-formed opening device on said hole of said web of packaging material, said application station comprising o a sonotrode which can be activated to cause the sealing of said pre-formed opening device to said web of packaging material; and o an anvil, which is adapted to co-operate with said pre-formed opening device, on the opposite side of said sonotrode with respect to said web, during the sealing of said pre-formed opening device to said web, o a sonotrode which can be activated to cause the sealing of said pre-formed opening device to said web of packaging material; and o an anvil, which is adapted to co-operate with said pre-formed opening device, on the opposite side of said sonotrode with respect to said web, during the sealing of said pre-formed opening device to said web, - said sonotrode is movable along a first direction between ∘ a first rest position, in which it is spaced apart from said web; and ∘ a first operative position, in which it cooperates with a first side of said web at said hole, and in which it is activated to cause the sealing of said pre-formed opening device; ∘ a first rest position, in which it is spaced apart from said web; and ∘ a first operative position, in which it cooperates with a first side of said web at said hole, and in which it is activated to cause the sealing of said pre-formed opening device; - said anvil is movable along a second direction transversal to said first direction between ∘ a second rest position, in which it is spaced apart from said web; and ∘ a second operative position, in which it exerts a counter-pressure against a second side, opposite to said first side, of said web at said hole; and ∘ a second rest position, in which it is spaced apart from said web; and ∘ a second operative position, in which it exerts a counter-pressure against a second side, opposite to said first side, of said web at said hole; - said sonotrode and said anvil are arranged, in use, at the same time, in respective said second operative positions, so as to seal said pre-formed opening device to said web.