The invention relates to an improved capsule for substances to be mixed at the time of use. The capsule object of the present invention is applied to close the mouth (1a) of a container (1) containing a solvent (10), and comprises a first part (2), of the known type, intended to be connected to the mouth of the container, and a second part (3), connected to the first part, by a wall (4) which firmly closes the bottom (3a) of the second part (3) and the mouth of the container intended to contain the active substance; the second part has an open upper mouth (3b), which is closed by hermetically removable sealing means when the second part contains the active substance.