A review of the four microstructures present in oil phases of malonamide extractants used for nuclear fuel reprocessing is presented in relation with the extraction properties and the third phase formation. Due to their surface active properties, extractants can be considered as a weak surfactants and are organised into reverse micelles interacting through an attractive potential. The Van der Waals attractions between the cores of reverse micelles are the key parameters for understanding “third phase” formation. Extractant solutions are similar to reverse microemulsions and the stability rules known in the field of microemulsions can be applied. In some cases, micelles are transformed into a regular molecular solution by modest heating. Apart from micellar and regular solutions, two new modes of association of extractant molecules have been identified. Networks of H-bounds are obtained when “modifiers” such as octanol are added by formulators in order to improve stability range. Microphase separation in a more crystalline phase may be obtained by increasing extractant and/or ion concentrations.