A process for modifying an organic natural substance by exchange reactions is characterised in that the natural substance to be modified is heated in the liquid phase as an aerosol in a gas stream, with the exclusion of air, along a conveyance path and, after reaching a temperature dependent on the bond energy of the molecules and/or molecular groups to be exchanged, is removed from the conveyance path to supply to the natural substance to be modified the materials required for the exchange reaction under the respectively required reaction conditions, and in that the reaction product is then supplied to the conveyance path in the circuit and heated to an elevated reaction temperature for carrying out the exchange reactions before the reaction product is withdrawn from the conveyance path and condensed to obtain the modified natural substance.