b Abstract: Organic reactivity in microorganized media is illustrated by results on micellar catalysis in micelles and on the properties of water in reverse micelles. With cationic micelles, interesting catalytic effects on reactions between hydrophobic and hydrophilic reagents are obtained when the hydrophilic partner, and only it, goes into the Stem layer by shifting the counter-ions of the surfactant. Results on nucleophilic substitution of a lipophilic phosphoric triester, PNDPP, by an oximate, MeCO-C(Me)=NO, in the presence of various cationic surfactants, cetyltrimethylammonium salts, CTAX, with X = Br', CI' and AcO , and of various buffers, glycine or carbonate, exemplie how the ion exchanges at the micellar interface control the interfacial nucleophile concentration and the reactivity. In anionic AOT-reverse micelles, alkene bromination, a water-promoted reaction between two lipophilic reagents, is taken as a probe for the chemical properties of the sodium-bound water of these microaggregates. Kinetic data show that the electrophilicity of water, i.e. its hydrogen-bond donor ability, is increased by a factor of up to 65 as compared to bulk water. Within the bound-water phase a gradient of counter-ion concentration is observed since the electrophilicity increase depends on the probe location, close to the interfacial layer or to the micellar core. Product data give evidence for a change in the nucleophilic reactivity of this unusual water. These results are discussed in terms of new routes for biomimetic chemistry