A unit for preparing infusions comprising: [a] a capsule comprising: [i] a truncated cup-shaped main body defining a main axis and having an upper base and an open lower base, [ii] a lid for closing said lower base and forming an inner chamber with said main body in which there is contained an infusion product, [iii] a perimetrical rim projecting outwardly from said lower base and having an injection side and an extraction side, and [iv] a sealing member, provided on said injection side of said rim, and [b] a device for preparing infusions comprising a truncated-cone-shaped enclosing member configured to receive said capsule and having a circular leading sealing edge, said device being configured to prepare an infusion by causing a liquid to pass under pressure through said inner chamber, said leading sealing edge having an annular cross section defining an inner diameter and an outer diameter, [c] said unit having a rest position in which said leading sealing edge is remote from said sealing member and an operative position in which said leading sealing edge is compressed against said sealing member to provide fluid tightness during the preparation of said infusion, [d] said sealing member comprising an annular groove surrounding the main body and which is formed by the side wall of said main body and a remote wall from said main body, said groove narrowing from the injection side towards the extraction side, said groove having a floor and said remote wall sloping between 15 and 45° relative to said main axis [e] said groove and said leading sealing edge being configured in such a way that, in the operative position, said leading sealing edge is inserted in said groove and is sealingly compressed simultaneously on said side wall of said main body and said remote wall, characterised in that [f] said leading edge has an inner diameter comprised between 29.8 and 30.4 mm and an outer diameter comprised between 30.6 and 32 mm, in that [g] said floor of said groove defines a floor diameter and said floor diameter is [i] greater than said inner diameter of the enclosing member and [ii] smaller than said outer diameter of the enclosing member, [h] said side wall in the region of said groove slopes between 14 and 17.5° relative to said main axis, and in that [i] said groove has a maximum height comprised between 0.8 and 3 mm measured from the lowest point of said extraction side and a depth comprised between 0.5 and 1.5 mm measured from said maximum height, to the floor of said groove. [a] a capsule comprising: [i] a truncated cup-shaped main body defining a main axis and having an upper base and an open lower base, [ii] a lid for closing said lower base and forming an inner chamber with said main body in which there is contained an infusion product, [iii] a perimetrical rim projecting outwardly from said lower base and having an injection side and an extraction side, and [iv] a sealing member, provided on said injection side of said rim, and [i] a truncated cup-shaped main body defining a main axis and having an upper base and an open lower base, [ii] a lid for closing said lower base and forming an inner chamber with said main body in which there is contained an infusion product, [iii] a perimetrical rim projecting outwardly from said lower base and having an injection side and an extraction side, and [iv] a sealing member, provided on said injection side of said rim, and [b] a device for preparing infusions comprising a truncated-cone-shaped enclosing member configured to receive said capsule and having a circular leading sealing edge, said device being configured to prepare an infusion by causing a liquid to pass under pressure through said inner chamber, said leading sealing edge having an annular cross section defining an inner diameter and an outer diameter, [c] said unit having a rest position in which said leading sealing edge is remote from said sealing member and an operative position in which said leading sealing edge is compressed against said sealing member to provide fluid tightness during the preparation of said infusion, [d] said sealing member comprising an annular groove surrounding the main body and which is formed by the side wall of said main body and a remote wall from said main body, said groove narrowing from the injection side towards the extraction side, said groove having a floor and said remote wall sloping between 15 and 45° relative to said main axis [e] said groove and said leading sealing edge being configured in such a way that, in the operative position, said leading sealing edge is inserted in said groove and is sealingly compressed simultaneously on said side wall of said main body and said remote wall, characterised in that [f] said leading edge has an inner diameter comprised between 29.8 and 30.4 mm and an outer diameter comprised between 30.6 and 32 mm, in that [g] said floor of said groove defines a floor diameter and said floor diameter is [i] greater than said inner diameter of the enclosing member and [ii] smaller than said outer diameter of the enclosing member, [i] greater than said inner diameter of the enclosing member and [ii] smaller than said outer diameter of the enclosing member, [h] said side wall in the region of said groove slopes between 14 and 17.5° relative to said main axis, and in that [i] said groove has a maximum height comprised between 0.8 and 3 mm measured from the lowest point of said extraction side and a depth comprised between 0.5 and 1.5 mm measured from said maximum height, to the floor of said groove.