The invention relates to a method for the preparation of soluble molecular complexes, comprising one or more active substances which are poorly-soluble in an aqueous medium, included within one or more host molecules, characterized in comprising the following steps: (a) bringing one or more active substances into contact with one or more host molecules, (b) carrying out a molecular diffusion step by bringing a dense fluid into contact, under pressure, with the mixture obtained in (a), in static mode, in the presence of one or more diffusion agents and (c) recovery of the molecular complex thus formed.