The invention relates to a diffusing apparatus for diffusing a substance in the vapor state into the surrounding air, and in particular to a removable assembly of said apparatus which includes a storage container (10, 110, 210) including an orifice (12) and containing a liquid substance selected from semiochemical molecules, pheromones, allomones, kairomones, synomones and fragrances of natural or synthetic origin, and an inner cellular retaining member housed at least in a mouth area of the storage container which is adjacent to the opening and set back from the opening. The removable assembly of the apparatus may include a dispensing member (30) including a porous body having an evaporation surface located outside the storage container. The substance may have a viscosity which varies according to temperature, the viscosity and the surface tension of the liquid substance being such that when the acceleration due to gravity is exerted from the opening toward the porous body, the substance does not flow through the porous body at an ambient temperature below a first temperature.