The invention concerns a capture mass for capturing heavy metals in a liquid or gaseous effluent, comprising a porous solid support, copper sulphide and at least one second metal sulphide the metal of which is selected from the group constituted by chromium, manganese, iron, cobalt and nickel, and in which the ratio of the percentage by weight of the metal or metals other than copper to the percentage by weight of copper is in the range 0.01 to 2. The invention also concerns a process for preparing said capture mass and a process for capturing heavy metals in a gaseous or liquid effluent, in which said effluent is brought into contact with said capture mass.